diff --git a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/ByteSseEntityConsumer.java b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/ByteSseEntityConsumer.java
index d3caa0867f..7a8225bd96 100644
--- a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/ByteSseEntityConsumer.java
+++ b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/ByteSseEntityConsumer.java
@@ -48,7 +48,7 @@
  *   <li>Validates {@code Content-Type} equals {@code text/event-stream}
  *       in {@link #streamStart(ContentType)}; otherwise throws {@link HttpException}.</li>
  *   <li>Strips a UTF-8 BOM if present in the first chunk.</li>
- *   <li>Accepts LF and CRLF line endings; tolerates CRLF split across buffers.</li>
+ *   <li>Accepts CR, LF and CRLF line endings per the SSE grammar; tolerates CRLF split across buffers.</li>
  *   <li>Implements WHATWG SSE fields: {@code data}, {@code id}, {@code event}, {@code retry}.
  *       Unknown fields and malformed {@code retry} values are ignored.</li>
  *   <li>At end of stream, flushes any partially accumulated line and forces a final
@@ -76,6 +76,7 @@ public final class ByteSseEntityConsumer extends AbstractBinAsyncEntityConsumer<
     // line accumulator
     private byte[] lineBuf = new byte[256];
     private int lineLen = 0;
+    private boolean pendingCr = false;
 
     // event accumulator
     private final StringBuilder data = new StringBuilder(256);
@@ -115,12 +116,12 @@ protected void data(final ByteBuffer src, final boolean endOfStream) {
                     continue;
                 }
                 if (bomMatched > 0) {
-                    appendByte((byte) 0xEF);
+                    processByte((byte) 0xEF);
                     if (bomMatched >= 2) {
-                        appendByte((byte) 0xBB);
+                        processByte((byte) 0xBB);
                     }
                 }
-                appendByte((byte) b);
+                processByte((byte) b);
                 bomMatched = 0;
                 bomDone = true;
                 break; // drop into normal loop below for the rest of 'src'
@@ -134,17 +135,7 @@ protected void data(final ByteBuffer src, final boolean endOfStream) {
         }
 
         while (src.hasRemaining()) {
-            final byte b = src.get();
-            if (b == LF) {
-                int len = lineLen;
-                if (len > 0 && lineBuf[len - 1] == CR) {
-                    len--;
-                }
-                handleLine(lineBuf, len);
-                lineLen = 0;
-            } else {
-                appendByte(b);
-            }
+            processByte(src.get());
         }
 
         if (endOfStream) {
@@ -152,13 +143,29 @@ protected void data(final ByteBuffer src, final boolean endOfStream) {
         }
     }
 
+    private void processByte(final byte b) {
+        if (b == LF) {
+            if (pendingCr) {
+                // LF completing a CRLF pair; the line was already emitted on the CR.
+                pendingCr = false;
+            } else {
+                handleLine(lineBuf, lineLen);
+                lineLen = 0;
+            }
+        } else if (b == CR) {
+            // A lone CR is a line terminator per the SSE grammar (CR / LF / CRLF).
+            pendingCr = true;
+            handleLine(lineBuf, lineLen);
+            lineLen = 0;
+        } else {
+            pendingCr = false;
+            appendByte(b);
+        }
+    }
+
     private void flushEndOfStream() {
         if (lineLen > 0) {
-            int len = lineLen;
-            if (lineBuf[len - 1] == CR) {
-                len--;
-            }
-            handleLine(lineBuf, len);
+            handleLine(lineBuf, lineLen);
             lineLen = 0;
         }
         handleLine(lineBuf, 0);
@@ -182,6 +189,8 @@ protected Void generateContent() {
     @Override
     public void releaseResources() {
         lineBuf = new byte[0];
+        lineLen = 0;
+        pendingCr = false;
         data.setLength(0);
         id = null;
         type = null;
diff --git a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/SseEntityConsumer.java b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/SseEntityConsumer.java
index 5b90d4e72a..3e8327b0bd 100644
--- a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/SseEntityConsumer.java
+++ b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/SseEntityConsumer.java
@@ -66,6 +66,7 @@ public final class SseEntityConsumer extends AbstractCharAsyncEntityConsumer<Voi
     private final StringBuilder partial = new StringBuilder(256);
     private ServerSentEventReader reader;
     private boolean firstChunk = true;
+    private boolean pendingCr = false;
 
     public SseEntityConsumer(final SseCallbacks callbacks) {
         this.cb = callbacks;
@@ -94,13 +95,20 @@ public void data(final CharBuffer src, final boolean endOfStream) {
         while (src.hasRemaining()) {
             final char c = src.get();
             if (c == '\n') {
-                final int len = partial.length();
-                if (len > 0 && partial.charAt(len - 1) == '\r') {
-                    partial.setLength(len - 1);
+                if (pendingCr) {
+                    // LF completing a CRLF pair; the line was already emitted on the CR.
+                    pendingCr = false;
+                } else {
+                    reader.line(partial.toString());
+                    partial.setLength(0);
                 }
+            } else if (c == '\r') {
+                // A lone CR is a line terminator per the SSE grammar (CR / LF / CRLF).
+                pendingCr = true;
                 reader.line(partial.toString());
                 partial.setLength(0);
             } else {
+                pendingCr = false;
                 partial.append(c);
             }
         }
@@ -127,6 +135,7 @@ protected Void generateContent() {
     @Override
     public void releaseResources() {
         partial.setLength(0);
+        pendingCr = false;
         reader = null;
     }
 
diff --git a/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/ByteSseEntityConsumerTest.java b/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/ByteSseEntityConsumerTest.java
index 4b0ceefc73..632409a6a5 100644
--- a/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/ByteSseEntityConsumerTest.java
+++ b/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/ByteSseEntityConsumerTest.java
@@ -84,6 +84,39 @@ void handlesBomCrLfAndDispatch() throws Exception {
         assertEquals("hi", cb.data);
     }
 
+    @Test
+    void treatsLoneCrAsLineTerminator() throws Exception {
+        final Cb cb = new Cb();
+        final ByteSseEntityConsumer c = new ByteSseEntityConsumer(cb);
+        c.streamStart(ContentType.parse("text/event-stream"));
+
+        // A stream delimited with lone CR (a valid SSE separator). A CR in the id value must
+        // terminate the line rather than be retained, otherwise it would later be copied into
+        // the Last-Event-ID request header on reconnect.
+        final byte[] p = "id: 1\rdata: v\r\r".getBytes(StandardCharsets.UTF_8);
+        c.consume(ByteBuffer.wrap(p));
+        c.streamEnd(null);
+
+        assertEquals("1", cb.id);
+        assertEquals("v", cb.data);
+    }
+
+    @Test
+    void treatsLeadingLoneCrAsLineTerminator() throws Exception {
+        final Cb cb = new Cb();
+        final ByteSseEntityConsumer c = new ByteSseEntityConsumer(cb);
+        c.streamStart(ContentType.parse("text/event-stream"));
+
+        // The first byte is a lone CR, which is resolved during BOM detection. It must still be
+        // routed through the normal CR/LF handling so the empty leading line is terminated rather
+        // than the CR being retained inside the buffer.
+        final byte[] p = "\rdata: v\r\r".getBytes(StandardCharsets.UTF_8);
+        c.consume(ByteBuffer.wrap(p));
+        c.streamEnd(null);
+
+        assertEquals("v", cb.data);
+    }
+
     @Test
     void emitsRetry() throws Exception {
         final Cb cb = new Cb();
diff --git a/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/SseEntityConsumerTest.java b/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/SseEntityConsumerTest.java
index bafdd4ed12..5dd5a6a2f7 100644
--- a/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/SseEntityConsumerTest.java
+++ b/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/SseEntityConsumerTest.java
@@ -77,6 +77,21 @@ void parsesLinesAndFlushesOnEndOfStream() throws Exception {
         assertEquals("v", cb.data);
     }
 
+    @Test
+    void treatsLoneCrAsLineTerminator() throws Exception {
+        final Cb cb = new Cb();
+        final SseEntityConsumer c = new SseEntityConsumer(cb);
+
+        c.streamStart(ContentType.parse("text/event-stream"));
+        // A stream delimited with lone CR (a valid SSE separator). A CR in the id value must
+        // terminate the line rather than be retained, otherwise it would later be copied into
+        // the Last-Event-ID request header on reconnect.
+        c.data(CharBuffer.wrap("id: 1\rdata: v\r\r"), true);
+
+        assertEquals("1", cb.id);
+        assertEquals("v", cb.data);
+    }
+
     @Test
     void rejectsWrongContentType() {
         final Cb cb = new Cb();