diff --git a/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GatherAllPanes.java b/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GatherAllPanes.java
new file mode 100644
index 0000000000..1d675e3487
--- /dev/null
+++ b/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GatherAllPanes.java
@@ -0,0 +1,72 @@
+/*
+ * Copyright (C) 2016 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.google.cloud.dataflow.sdk.util;
+
+import com.google.cloud.dataflow.sdk.transforms.GroupByKey;
+import com.google.cloud.dataflow.sdk.transforms.GroupByKey.ReifyTimestampsAndWindows;
+import com.google.cloud.dataflow.sdk.transforms.PTransform;
+import com.google.cloud.dataflow.sdk.transforms.Values;
+import com.google.cloud.dataflow.sdk.transforms.WithKeys;
+import com.google.cloud.dataflow.sdk.transforms.windowing.Never;
+import com.google.cloud.dataflow.sdk.transforms.windowing.Window;
+import com.google.cloud.dataflow.sdk.transforms.windowing.WindowFn;
+import com.google.cloud.dataflow.sdk.values.KV;
+import com.google.cloud.dataflow.sdk.values.PCollection;
+import com.google.cloud.dataflow.sdk.values.TypeDescriptor;
+
+/**
+ * Gathers all panes of each window into exactly one output.
+ *
+ * <p>Note that this will delay the output of a window until the garbage collection time (when the
+ * watermark passes the end of the window plus allowed lateness) even if the upstream triggers
+ * closed the window earlier.
+ */
+public class GatherAllPanes<T>
+    extends PTransform<PCollection<T>, PCollection<Iterable<WindowedValue<T>>>> {
+  /**
+   * Gathers all panes of each window into a single output element.
+   *
+   * <p>This will gather all output panes into a single element, which causes them to be colocated
+   * on a single worker. As a result, this is only suitable for {@link PCollection PCollections}
+   * where all of the output elements for each pane fit in memory, such as in tests.
+   */
+  public static <T> GatherAllPanes<T> globally() {
+    return new GatherAllPanes<>();
+  }
+
+  private GatherAllPanes() {}
+
+  @Override
+  public PCollection<Iterable<WindowedValue<T>>> apply(PCollection<T> input) {
+    WindowFn<?, ?> originalWindowFn = input.getWindowingStrategy().getWindowFn();
+
+    return input
+        .apply(WithKeys.<Integer, T>of(0).withKeyType(new TypeDescriptor<Integer>() {}))
+        .apply(new ReifyTimestampsAndWindows<Integer, T>())
+        .apply(
+            Window.into(
+                new IdentityWindowFn<KV<Integer, WindowedValue<T>>>(
+                    originalWindowFn.windowCoder(), originalWindowFn.assignsToSingleWindow()))
+                .triggering(Never.ever())
+                .withAllowedLateness(input.getWindowingStrategy().getAllowedLateness())
+                .discardingFiredPanes())
+        // all values have the same key so they all appear as a single output element
+        .apply(GroupByKey.<Integer, WindowedValue<T>>create())
+        .apply(Values.<Iterable<WindowedValue<T>>>create())
+        .setWindowingStrategyInternal(input.getWindowingStrategy());
+  }
+}
+
diff --git a/sdk/src/test/java/com/google/cloud/dataflow/sdk/util/GatherAllPanesTest.java b/sdk/src/test/java/com/google/cloud/dataflow/sdk/util/GatherAllPanesTest.java
new file mode 100644
index 0000000000..37bf04d469
--- /dev/null
+++ b/sdk/src/test/java/com/google/cloud/dataflow/sdk/util/GatherAllPanesTest.java
@@ -0,0 +1,129 @@
+/*
+ * Copyright (C) 2016 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.google.cloud.dataflow.sdk.util;
+
+import static org.junit.Assert.fail;
+
+import com.google.cloud.dataflow.sdk.io.CountingInput;
+import com.google.cloud.dataflow.sdk.testing.DataflowAssert;
+import com.google.cloud.dataflow.sdk.testing.TestPipeline;
+import com.google.cloud.dataflow.sdk.transforms.GroupByKey;
+import com.google.cloud.dataflow.sdk.transforms.SerializableFunction;
+import com.google.cloud.dataflow.sdk.transforms.Values;
+import com.google.cloud.dataflow.sdk.transforms.WithKeys;
+import com.google.cloud.dataflow.sdk.transforms.WithTimestamps;
+import com.google.cloud.dataflow.sdk.transforms.windowing.AfterPane;
+import com.google.cloud.dataflow.sdk.transforms.windowing.AfterWatermark;
+import com.google.cloud.dataflow.sdk.transforms.windowing.FixedWindows;
+import com.google.cloud.dataflow.sdk.transforms.windowing.Window;
+import com.google.cloud.dataflow.sdk.values.PCollection;
+import com.google.cloud.dataflow.sdk.values.TypeDescriptor;
+import com.google.common.collect.Iterables;
+
+import org.joda.time.Duration;
+import org.joda.time.Instant;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+import java.io.Serializable;
+
+/**
+ * Tests for {@link GatherAllPanes}.
+ */
+@RunWith(JUnit4.class)
+public class GatherAllPanesTest implements Serializable {
+  @Test
+  public void singlePaneSingleReifiedPane() {
+    TestPipeline p = TestPipeline.create();
+    PCollection<Iterable<WindowedValue<Iterable<Long>>>> accumulatedPanes =
+        p.apply(CountingInput.upTo(20000))
+            .apply(
+                WithTimestamps.of(
+                    new SerializableFunction<Long, Instant>() {
+                      @Override
+                      public Instant apply(Long input) {
+                        return new Instant(input * 10);
+                      }
+                    }))
+            .apply(
+                Window.<Long>into(FixedWindows.of(Duration.standardMinutes(1)))
+                    .triggering(AfterWatermark.pastEndOfWindow())
+                    .withAllowedLateness(Duration.ZERO)
+                    .discardingFiredPanes())
+            .apply(WithKeys.<Void, Long>of((Void) null).withKeyType(new TypeDescriptor<Void>() {}))
+            .apply(GroupByKey.<Void, Long>create())
+            .apply(Values.<Iterable<Long>>create())
+            .apply(GatherAllPanes.<Iterable<Long>>globally());
+
+    DataflowAssert.that(accumulatedPanes)
+        .satisfies(
+            new SerializableFunction<Iterable<Iterable<WindowedValue<Iterable<Long>>>>, Void>() {
+              @Override
+              public Void apply(Iterable<Iterable<WindowedValue<Iterable<Long>>>> input) {
+                for (Iterable<WindowedValue<Iterable<Long>>> windowedInput : input) {
+                  if (Iterables.size(windowedInput) > 1) {
+                    fail("Expected all windows to have exactly one pane, got " + windowedInput);
+                    return null;
+                  }
+                }
+                return null;
+              }
+            });
+  }
+
+  @Test
+  public void multiplePanesMultipleReifiedPane() {
+    TestPipeline p = TestPipeline.create();
+
+    PCollection<Iterable<WindowedValue<Iterable<Long>>>> accumulatedPanes =
+        p.apply(CountingInput.upTo(20000))
+            .apply(
+                WithTimestamps.of(
+                    new SerializableFunction<Long, Instant>() {
+                      @Override
+                      public Instant apply(Long input) {
+                        return new Instant(input * 10);
+                      }
+                    }))
+            .apply(
+                Window.<Long>into(FixedWindows.of(Duration.standardMinutes(1)))
+                    .triggering(
+                        AfterWatermark.pastEndOfWindow()
+                            .withEarlyFirings(AfterPane.elementCountAtLeast(1)))
+                    .withAllowedLateness(Duration.ZERO)
+                    .discardingFiredPanes())
+            .apply(WithKeys.<Void, Long>of((Void) null).withKeyType(new TypeDescriptor<Void>() {}))
+            .apply(GroupByKey.<Void, Long>create())
+            .apply(Values.<Iterable<Long>>create())
+            .apply(GatherAllPanes.<Iterable<Long>>globally());
+
+    DataflowAssert.that(accumulatedPanes)
+        .satisfies(
+            new SerializableFunction<Iterable<Iterable<WindowedValue<Iterable<Long>>>>, Void>() {
+              @Override
+              public Void apply(Iterable<Iterable<WindowedValue<Iterable<Long>>>> input) {
+                for (Iterable<WindowedValue<Iterable<Long>>> windowedInput : input) {
+                  if (Iterables.size(windowedInput) > 1) {
+                    return null;
+                  }
+                }
+                fail("Expected at least one window to have multiple panes");
+                return null;
+              }
+            });
+  }
+}