diff --git a/adapters/src/main/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactory.java b/adapters/src/main/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactory.java
new file mode 100644
index 0000000..44b97d2
--- /dev/null
+++ b/adapters/src/main/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactory.java
@@ -0,0 +1,158 @@
+/*
+ * Copyright (C) 2011 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.moshi.adapters;
+
+import com.squareup.moshi.JsonAdapter;
+import com.squareup.moshi.JsonDataException;
+import com.squareup.moshi.JsonReader;
+import com.squareup.moshi.JsonWriter;
+import com.squareup.moshi.Moshi;
+import com.squareup.moshi.Types;
+import java.io.IOException;
+import java.lang.annotation.Annotation;
+import java.lang.reflect.Type;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.Set;
+import javax.annotation.CheckReturnValue;
+
+/**
+ * A JsonAdapter factory for polymorphic types. This is useful when the type is not known before
+ * deserializing the JSON. This factory's adapters expect JSON in the format of a JSON object with a
+ * key whose value is a label that determines the type to which to map the JSON object.
+ */
+public final class RuntimeJsonAdapterFactory<T> implements JsonAdapter.Factory {
+  final Class<T> baseType;
+  final String labelKey;
+  final Map<String, Type> labelToType = new LinkedHashMap<>();
+
+  /**
+   * @param baseType The base type for which this factory will create adapters.
+   * @param labelKey The key in the JSON object whose value determines the type to which to map the
+   * JSON object.
+   */
+  @CheckReturnValue
+  public static <T> RuntimeJsonAdapterFactory<T> of(Class<T> baseType, String labelKey) {
+    if (baseType == null) throw new NullPointerException("baseType == null");
+    if (labelKey == null) throw new NullPointerException("labelKey == null");
+    return new RuntimeJsonAdapterFactory<>(baseType, labelKey);
+  }
+
+  RuntimeJsonAdapterFactory(Class<T> baseType, String labelKey) {
+    this.baseType = baseType;
+    this.labelKey = labelKey;
+  }
+
+  /**
+   * Register the subtype that can be created based on the label. When deserializing, if a label
+   * that was not registered is found, a JsonDataException will be thrown. When serializing, if a
+   * type that was not registered is used, an IllegalArgumentException will be thrown.
+   */
+  public RuntimeJsonAdapterFactory<T> registerSubtype(Class<? extends T> subtype, String label) {
+    if (subtype == null) throw new NullPointerException("subtype == null");
+    if (label == null) throw new NullPointerException("label == null");
+    if (labelToType.containsKey(label) || labelToType.containsValue(subtype)) {
+      throw new IllegalArgumentException("Subtypes and labels must be unique.");
+    }
+    labelToType.put(label, subtype);
+    return this;
+  }
+
+  @Override
+  public JsonAdapter<?> create(Type type, Set<? extends Annotation> annotations, Moshi moshi) {
+    if (Types.getRawType(type) != baseType || !annotations.isEmpty()) {
+      return null;
+    }
+    int size = labelToType.size();
+    Map<Type, JsonAdapter<Object>> typeToAdapter = new LinkedHashMap<>(size);
+    Map<String, JsonAdapter<Object>> labelToAdapter = new LinkedHashMap<>(size);
+    for (Map.Entry<String, Type> entry : labelToType.entrySet()) {
+      String label = entry.getKey();
+      Type typeValue = entry.getValue();
+      JsonAdapter<Object> adapter = moshi.adapter(typeValue);
+      labelToAdapter.put(label, adapter);
+      typeToAdapter.put(typeValue, adapter);
+    }
+    return new RuntimeJsonAdapter(labelKey, labelToAdapter, typeToAdapter).nullSafe();
+  }
+
+  static final class RuntimeJsonAdapter extends JsonAdapter<Object> {
+    final String labelKey;
+    final Map<String, JsonAdapter<Object>> labelToAdapter;
+    final Map<Type, JsonAdapter<Object>> typeToAdapter;
+
+    RuntimeJsonAdapter(
+        String labelKey,
+        Map<String, JsonAdapter<Object>> labelToAdapter,
+        Map<Type, JsonAdapter<Object>> typeToAdapter) {
+      this.labelKey = labelKey;
+      this.labelToAdapter = labelToAdapter;
+      this.typeToAdapter = typeToAdapter;
+    }
+
+    @Override public Object fromJson(JsonReader reader) throws IOException {
+      JsonReader.Token peekedToken = reader.peek();
+      if (peekedToken != JsonReader.Token.BEGIN_OBJECT) {
+        throw new JsonDataException("Expected BEGIN_OBJECT but was " + peekedToken
+            + " at path " + reader.getPath());
+      }
+      Object jsonValue = reader.readJsonValue();
+      Map<String, Object> jsonObject = (Map<String, Object>) jsonValue;
+      Object label = jsonObject.get(labelKey);
+      if (label == null) {
+        throw new JsonDataException("Missing label for " + labelKey);
+      }
+      if (!(label instanceof String)) {
+        throw new JsonDataException("Label for '"
+            + labelKey
+            + "' must be a string but was "
+            + label
+            + ", a "
+            + label.getClass());
+      }
+      JsonAdapter<Object> adapter = labelToAdapter.get(label);
+      if (adapter == null) {
+        throw new JsonDataException("Expected one of "
+            + labelToAdapter.keySet()
+            + " for key '"
+            + labelKey
+            + "' but found '"
+            + label
+            + "'. Register a subtype for this label.");
+      }
+      return adapter.fromJsonValue(jsonValue);
+    }
+
+    @Override public void toJson(JsonWriter writer, Object value) throws IOException {
+      Class<?> type = value.getClass();
+      JsonAdapter<Object> adapter = typeToAdapter.get(type);
+      if (adapter == null) {
+        throw new IllegalArgumentException("Expected one of "
+            + typeToAdapter.keySet()
+            + " but found "
+            + value
+            + ", a "
+            + value.getClass()
+            + ". Register this subtype.");
+      }
+      adapter.toJson(writer, value);
+    }
+
+    @Override public String toString() {
+      return "RuntimeJsonAdapter(" + labelKey + ")";
+    }
+  }
+}
diff --git a/adapters/src/test/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactoryTest.java b/adapters/src/test/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactoryTest.java
new file mode 100644
index 0000000..a105fd5
--- /dev/null
+++ b/adapters/src/test/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactoryTest.java
@@ -0,0 +1,220 @@
+/*
+ * Copyright (C) 2018 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.moshi.adapters;
+
+import com.squareup.moshi.JsonAdapter;
+import com.squareup.moshi.JsonDataException;
+import com.squareup.moshi.JsonReader;
+import com.squareup.moshi.Moshi;
+import java.io.IOException;
+import java.util.Collections;
+import java.util.Map;
+import okio.Buffer;
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.fail;
+
+@SuppressWarnings("CheckReturnValue")
+public final class RuntimeJsonAdapterFactoryTest {
+  @Test public void fromJson() throws IOException {
+    Moshi moshi = new Moshi.Builder()
+        .add(RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "success")
+            .registerSubtype(Error.class, "error"))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    assertThat(adapter.fromJson("{\"type\":\"success\",\"value\":\"Okay!\"}"))
+        .isEqualTo(new Success("Okay!"));
+    assertThat(adapter.fromJson("{\"type\":\"error\",\"error_logs\":{\"order\":66}}"))
+        .isEqualTo(new Error(Collections.<String, Object>singletonMap("order", 66d)));
+  }
+
+  @Test public void toJson() {
+    Moshi moshi = new Moshi.Builder()
+        .add(RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "success")
+            .registerSubtype(Error.class, "error"))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    assertThat(adapter.toJson(new Success("Okay!")))
+        .isEqualTo("{\"value\":\"Okay!\"}");
+    assertThat(adapter.toJson(new Error(Collections.<String, Object>singletonMap("order", 66))))
+        .isEqualTo("{\"error_logs\":{\"order\":66}}");
+  }
+
+  @Test public void unregisteredLabelValue() throws IOException {
+    Moshi moshi = new Moshi.Builder()
+        .add(RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "success")
+            .registerSubtype(Error.class, "error"))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    JsonReader reader =
+        JsonReader.of(new Buffer().writeUtf8("{\"type\":\"data\",\"value\":\"Okay!\"}"));
+    try {
+      adapter.fromJson(reader);
+      fail();
+    } catch (JsonDataException expected) {
+      assertThat(expected).hasMessage("Expected one of [success, error] for key 'type' but found"
+          + " 'data'. Register a subtype for this label.");
+    }
+    assertThat(reader.peek()).isEqualTo(JsonReader.Token.END_DOCUMENT);
+  }
+
+  @Test public void unregisteredSubtype() {
+    Moshi moshi = new Moshi.Builder()
+        .add(RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "success")
+            .registerSubtype(Error.class, "error"))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    try {
+      adapter.toJson(new EmptyMessage());
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("Expected one of [class"
+          + " com.squareup.moshi.adapters.RuntimeJsonAdapterFactoryTest$Success, class"
+          + " com.squareup.moshi.adapters.RuntimeJsonAdapterFactoryTest$Error] but found"
+          + " EmptyMessage, a class"
+          + " com.squareup.moshi.adapters.RuntimeJsonAdapterFactoryTest$EmptyMessage. Register"
+          + " this subtype.");
+    }
+  }
+
+  @Test public void nonStringLabelValue() throws IOException {
+    Moshi moshi = new Moshi.Builder()
+        .add(RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "success")
+            .registerSubtype(Error.class, "error"))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    try {
+      adapter.fromJson("{\"type\":{},\"value\":\"Okay!\"}");
+      fail();
+    } catch (JsonDataException expected) {
+      assertThat(expected).hasMessage("Label for 'type' must be a string but was {},"
+          + " a class com.squareup.moshi.LinkedHashTreeMap");
+    }
+  }
+
+  @Test public void nonObjectDoesNotConsume() throws IOException {
+    Moshi moshi = new Moshi.Builder()
+        .add(RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "success")
+            .registerSubtype(Error.class, "error"))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    JsonReader reader = JsonReader.of(new Buffer().writeUtf8("\"Failure\""));
+    try {
+      adapter.fromJson(reader);
+      fail();
+    } catch (JsonDataException expected) {
+      assertThat(expected).hasMessage("Expected BEGIN_OBJECT but was STRING at path $");
+    }
+    assertThat(reader.nextString()).isEqualTo("Failure");
+    assertThat(reader.peek()).isEqualTo(JsonReader.Token.END_DOCUMENT);
+  }
+
+  @Test public void uniqueSubtypes() {
+    RuntimeJsonAdapterFactory<Message> factory =
+        RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "success");
+    try {
+      factory.registerSubtype(Success.class, "data");
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("Subtypes and labels must be unique.");
+    }
+  }
+
+  @Test public void uniqueLabels() {
+    RuntimeJsonAdapterFactory<Message> factory =
+        RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "data");
+    try {
+      factory.registerSubtype(Error.class, "data");
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("Subtypes and labels must be unique.");
+    }
+  }
+
+  @Test public void nullSafe() throws IOException {
+    Moshi moshi = new Moshi.Builder()
+        .add(RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .registerSubtype(Success.class, "success")
+            .registerSubtype(Error.class, "error"))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    JsonReader reader = JsonReader.of(new Buffer().writeUtf8("null"));
+    assertThat(adapter.fromJson(reader)).isNull();
+    assertThat(reader.peek()).isEqualTo(JsonReader.Token.END_DOCUMENT);
+  }
+
+  interface Message {
+  }
+
+  static final class Success implements Message {
+    final String value;
+
+    Success(String value) {
+      this.value = value;
+    }
+
+    @Override public boolean equals(Object o) {
+      if (this == o) return true;
+      if (!(o instanceof Success)) return false;
+      Success success = (Success) o;
+      return value.equals(success.value);
+    }
+
+    @Override public int hashCode() {
+      return value.hashCode();
+    }
+  }
+
+  static final class Error implements Message {
+    final Map<String, Object> error_logs;
+
+    Error(Map<String, Object> error_logs) {
+      this.error_logs = error_logs;
+    }
+
+    @Override public boolean equals(Object o) {
+      if (this == o) return true;
+      if (!(o instanceof Error)) return false;
+      Error error = (Error) o;
+      return error_logs.equals(error.error_logs);
+    }
+
+    @Override public int hashCode() {
+      return error_logs.hashCode();
+    }
+  }
+
+  static final class EmptyMessage implements Message {
+    @Override public String toString() {
+      return "EmptyMessage";
+    }
+  }
+}