diff --git a/spring-web/src/main/java/org/springframework/http/codec/ServerSentEventHttpMessageReader.java b/spring-web/src/main/java/org/springframework/http/codec/ServerSentEventHttpMessageReader.java
index 3966bcba7d70..1ca198f35098 100644
--- a/spring-web/src/main/java/org/springframework/http/codec/ServerSentEventHttpMessageReader.java
+++ b/spring-web/src/main/java/org/springframework/http/codec/ServerSentEventHttpMessageReader.java
@@ -163,7 +163,10 @@ else if (line.startsWith("event:")) {
 					sseBuilder.event(line.substring(6).trim());
 				}
 				else if (line.startsWith("retry:")) {
-					sseBuilder.retry(Duration.ofMillis(Long.parseLong(line.substring(6).trim())));
+					Long retry = parseRetry(line.substring(6).trim());
+					if (retry != null) {
+						sseBuilder.retry(Duration.ofMillis(retry));
+					}
 				}
 				else if (line.startsWith(":")) {
 					comment = (comment != null ? comment : new StringBuilder());
@@ -188,6 +191,32 @@ else if (line.startsWith(":")) {
 		}
 	}
 
+	/**
+	 * Parse the value of a {@code retry} field, which is to be ignored unless it
+	 * consists solely of ASCII digits and fits into a {@code long}.
+	 * @return the reconnection time in milliseconds, or {@code null} to ignore the field
+	 * @see <a href="https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation">
+	 * HTML Living Standard: interpreting an event stream</a>
+	 */
+	private static @Nullable Long parseRetry(String value) {
+		if (value.isEmpty()) {
+			return null;
+		}
+		for (int i = 0; i < value.length(); i++) {
+			char ch = value.charAt(i);
+			if (ch < '0' || ch > '9') {
+				return null;
+			}
+		}
+		try {
+			return Long.parseLong(value);
+		}
+		catch (NumberFormatException ex) {
+			// Too large for a long: ignore the field rather than failing the stream.
+			return null;
+		}
+	}
+
 	private @Nullable Object decodeData(StringBuilder data, ResolvableType dataType, Map<String, Object> hints) {
 		if (String.class == dataType.resolve()) {
 			return data.substring(0, data.length() - 1);
diff --git a/spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageReaderTests.java b/spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageReaderTests.java
index 3693bed3755d..7c704e5cf842 100644
--- a/spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageReaderTests.java
+++ b/spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageReaderTests.java
@@ -137,6 +137,29 @@ void trimWhitespace() {
 				.verify();
 	}
 
+	@Test
+	@SuppressWarnings("rawtypes")
+	void ignoreInvalidRetry() {
+		MockServerHttpRequest request = MockServerHttpRequest.post("/")
+				.body(Mono.just(stringBuffer(
+						"retry:none\ndata:foo\n\n" +
+								"retry:\ndata:bar\n\n" +
+								"retry:-1\ndata:baz\n\n" +
+								"retry:99999999999999999999\ndata:qux\n\n")));
+
+		Flux<ServerSentEvent> events = this.reader
+				.read(ResolvableType.forClassWithGenerics(ServerSentEvent.class, String.class),
+						request, Collections.emptyMap()).cast(ServerSentEvent.class);
+
+		StepVerifier.create(events)
+				.expectNext(ServerSentEvent.builder().data("foo").build())
+				.expectNext(ServerSentEvent.builder().data("bar").build())
+				.expectNext(ServerSentEvent.builder().data("baz").build())
+				.expectNext(ServerSentEvent.builder().data("qux").build())
+				.expectComplete()
+				.verify();
+	}
+
 	@Test // gh-35412
 	void emptyLines() {
 		MockServerHttpRequest request = MockServerHttpRequest.post("/")