diff --git a/spring-webflux/src/main/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketSession.java b/spring-webflux/src/main/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketSession.java
index 33e8d9cefcfc..84f6d056ec5c 100644
--- a/spring-webflux/src/main/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketSession.java
+++ b/spring-webflux/src/main/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketSession.java
@@ -209,7 +209,7 @@ protected Action process() {
 							}
 
 							ByteBuffer buffer = iterator.next();
-							boolean last = iterator.hasNext();
+							boolean last = !iterator.hasNext();
 							session.sendPartialBinary(buffer, last, Callback.from(this::succeeded, this::failed));
 							return Action.SCHEDULED;
 						}
diff --git a/spring-webflux/src/test/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketSessionTests.java b/spring-webflux/src/test/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketSessionTests.java
new file mode 100644
index 000000000000..5f41dfac45f1
--- /dev/null
+++ b/spring-webflux/src/test/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketSessionTests.java
@@ -0,0 +1,144 @@
+/*
+ * Copyright 2002-present the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.web.reactive.socket.adapter;
+
+import java.net.URI;
+import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+import org.eclipse.jetty.websocket.api.Callback;
+import org.eclipse.jetty.websocket.api.Session;
+import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentCaptor;
+import reactor.core.publisher.Mono;
+
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DataBufferWrapper;
+import org.springframework.core.io.buffer.DefaultDataBufferFactory;
+import org.springframework.http.HttpHeaders;
+import org.springframework.web.reactive.socket.HandshakeInfo;
+import org.springframework.web.reactive.socket.WebSocketMessage;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyBoolean;
+import static org.mockito.Mockito.doAnswer;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+/**
+ * Tests for {@link JettyWebSocketSession}.
+ *
+ * @author Max Guiking
+ */
+class JettyWebSocketSessionTests {
+
+	private final DefaultDataBufferFactory bufferFactory = DefaultDataBufferFactory.sharedInstance;
+
+	private final Session jettySession = mock(Session.class);
+
+	private final JettyWebSocketSession session = new JettyWebSocketSession(this.jettySession,
+			new HandshakeInfo(URI.create("ws://example.org"), new HttpHeaders(), Mono.empty(), null),
+			this.bufferFactory);
+
+
+	@Test
+	void sendBinaryMessageWithSingleFragmentMarksFragmentAsLast() {
+		succeedOnSendPartialBinary();
+
+		DataBuffer payload = this.bufferFactory.wrap("hello".getBytes(StandardCharsets.UTF_8));
+		WebSocketMessage message = new WebSocketMessage(WebSocketMessage.Type.BINARY, payload);
+
+		this.session.sendMessage(message).block();
+
+		ArgumentCaptor<Boolean> lastCaptor = ArgumentCaptor.forClass(Boolean.class);
+		verify(this.jettySession).sendPartialBinary(any(ByteBuffer.class), lastCaptor.capture(), any(Callback.class));
+		assertThat(lastCaptor.getValue()).as("FIN bit must be set for the final (and only) fragment").isTrue();
+	}
+
+	@Test
+	void sendBinaryMessageWithMultipleFragmentsMarksOnlyFinalFragmentAsLast() {
+		succeedOnSendPartialBinary();
+
+		List<ByteBuffer> fragments = List.of(
+				ByteBuffer.wrap("one".getBytes(StandardCharsets.UTF_8)),
+				ByteBuffer.wrap("two".getBytes(StandardCharsets.UTF_8)),
+				ByteBuffer.wrap("three".getBytes(StandardCharsets.UTF_8)));
+		WebSocketMessage message = new WebSocketMessage(WebSocketMessage.Type.BINARY,
+				new MultiBufferDataBuffer(this.bufferFactory, fragments));
+
+		this.session.sendMessage(message).block();
+
+		ArgumentCaptor<Boolean> lastCaptor = ArgumentCaptor.forClass(Boolean.class);
+		verify(this.jettySession, times(fragments.size()))
+				.sendPartialBinary(any(ByteBuffer.class), lastCaptor.capture(), any(Callback.class));
+		assertThat(lastCaptor.getAllValues()).containsExactly(false, false, true);
+	}
+
+	private void succeedOnSendPartialBinary() {
+		doAnswer(invocation -> {
+			Callback callback = invocation.getArgument(2);
+			callback.succeed();
+			return null;
+		}).when(this.jettySession).sendPartialBinary(any(ByteBuffer.class), anyBoolean(), any(Callback.class));
+	}
+
+
+	/**
+	 * Minimal {@link DataBuffer} whose {@link #readableByteBuffers()} yields a
+	 * caller-supplied list of buffers, exercising the multi-fragment branch of
+	 * {@link JettyWebSocketSession#sendMessage(WebSocketMessage)}.
+	 */
+	private static final class MultiBufferDataBuffer extends DataBufferWrapper {
+
+		private final List<ByteBuffer> buffers;
+
+		MultiBufferDataBuffer(DefaultDataBufferFactory factory, List<ByteBuffer> buffers) {
+			super(factory.allocateBuffer(0));
+			this.buffers = buffers;
+		}
+
+		@Override
+		public DataBuffer.ByteBufferIterator readableByteBuffers() {
+			return new DataBuffer.ByteBufferIterator() {
+
+				private int index = 0;
+
+				@Override
+				public boolean hasNext() {
+					return this.index < MultiBufferDataBuffer.this.buffers.size();
+				}
+
+				@Override
+				public ByteBuffer next() {
+					if (!hasNext()) {
+						throw new NoSuchElementException();
+					}
+					return MultiBufferDataBuffer.this.buffers.get(this.index++).asReadOnlyBuffer();
+				}
+
+				@Override
+				public void close() {
+				}
+			};
+		}
+	}
+
+}