diff --git a/changes.md b/changes.md
index d625a433ac..e5d8d3bdce 100644
--- a/changes.md
+++ b/changes.md
@@ -229,6 +229,8 @@ Changes log
           entity stream only, and is ignored after the head of a response announcing an empty
           body. The failures caused by stopping the client report "Client stopped". Issue #1601.
         - The `Max-Age` and `Path` attributes of a cookie setting are no longer quoted. Issue #1334.
+        - `MultiPartRepresentation.setBoundary` kept only the boundary parameter when replacing an
+          existing boundary, dropping the media type's other parameters. Issue #1600.
         - `Restlet.handle()` no longer fails with a `NullPointerException` when the Restlet has no context and cannot be started.
         - `CookieAuthenticator` no longer lets the next restlet overwrite the redirection sent after
           a login. Issue #554.
diff --git a/org.restlet/src/main/java/org/restlet/representation/MultiPartRepresentation.java b/org.restlet/src/main/java/org/restlet/representation/MultiPartRepresentation.java
index bd93fdbde1..8fa7448647 100644
--- a/org.restlet/src/main/java/org/restlet/representation/MultiPartRepresentation.java
+++ b/org.restlet/src/main/java/org/restlet/representation/MultiPartRepresentation.java
@@ -201,8 +201,8 @@ public static String getBoundary(MediaType mediaType) {
     }
 
     /**
-     * Sets a boundary to an existing media type. If the original media-type already has a
-     * "boundary" parameter, it will be erased. *
+     * Sets a boundary to an existing media type. If the original media type already has a
+     * "boundary" parameter, its value is replaced. The other parameters are kept.
      *
      * @param mediaType The media type to update.
      * @param boundary The boundary to add as a parameter.
@@ -212,13 +212,57 @@ public static MediaType setBoundary(MediaType mediaType, String boundary) {
         MediaType result = null;
 
         if (mediaType != null) {
-            if (mediaType.getParameters().getFirst(PARAMETER_BOUNDARY) != null) {
-                result = new MediaType(mediaType.getParent(), PARAMETER_BOUNDARY, boundary);
-            } else {
-                result = new MediaType(mediaType, PARAMETER_BOUNDARY, boundary);
+            result =
+                    new MediaType(
+                            new MediaType(
+                                    withoutBoundary(mediaType.getName()),
+                                    mediaType.getDescription()),
+                            PARAMETER_BOUNDARY,
+                            boundary);
+        }
+
+        return result;
+    }
+
+    /**
+     * Removes the boundary parameters from a media type name, which holds all the parameters,
+     * keeping the others as written. A null name, accepted by {@link MediaType}, is returned as is.
+     */
+    private static String withoutBoundary(String name) {
+        int index = (name == null) ? -1 : name.indexOf(';');
+        if (index < 0) {
+            return name;
+        }
+
+        StringBuilder result = new StringBuilder(name.substring(0, index));
+        for (String parameter : splitParameters(name.substring(index + 1))) {
+            String trimmed = parameter.trim();
+            int equals = trimmed.indexOf('=');
+            String parameterName = (equals < 0) ? trimmed : trimmed.substring(0, equals).trim();
+            if (!trimmed.isEmpty() && !PARAMETER_BOUNDARY.equalsIgnoreCase(parameterName)) {
+                result.append("; ").append(trimmed);
             }
         }
+        return result.toString();
+    }
 
+    /** Splits parameters on ';', except inside quoted strings. */
+    private static List<String> splitParameters(String parameters) {
+        List<String> result = new ArrayList<>();
+        boolean quoted = false;
+        int start = 0;
+        for (int i = 0; i < parameters.length(); i++) {
+            char c = parameters.charAt(i);
+            if (quoted && c == '\\') {
+                i++;
+            } else if (c == '"') {
+                quoted = !quoted;
+            } else if (c == ';' && !quoted) {
+                result.add(parameters.substring(start, i));
+                start = i + 1;
+            }
+        }
+        result.add(parameters.substring(start));
         return result;
     }
 
diff --git a/org.restlet/src/test/java/org/restlet/engine/connector/MultiPartRepresentationTestCase.java b/org.restlet/src/test/java/org/restlet/engine/connector/MultiPartRepresentationTestCase.java
index 866d4a67c8..6c5c8df2a7 100644
--- a/org.restlet/src/test/java/org/restlet/engine/connector/MultiPartRepresentationTestCase.java
+++ b/org.restlet/src/test/java/org/restlet/engine/connector/MultiPartRepresentationTestCase.java
@@ -20,10 +20,12 @@
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
 import org.restlet.data.MediaType;
+import org.restlet.data.Parameter;
 import org.restlet.representation.FileRepresentation;
 import org.restlet.representation.MultiPartRepresentation;
 import org.restlet.representation.MultiPartRepresentation.Part;
 import org.restlet.representation.StringRepresentation;
+import org.restlet.util.Series;
 
 /**
  * Test case for the {@link MultiPartRepresentation} class in multipart mode.
@@ -77,6 +79,58 @@ void testContentType() {
                 "multipart/form-data; boundary=myActualBoundary", rep.getMediaType().toString());
     }
 
+    @Test
+    void setBoundaryKeepsTheOtherParameters() {
+        assertEquals(
+                "multipart/mixed; deferSpec=20220824; boundary=b",
+                MultiPartRepresentation.setBoundary(
+                                MediaType.valueOf("multipart/mixed; deferSpec=20220824"), "b")
+                        .toString());
+        assertEquals(
+                "multipart/mixed; deferSpec=20220824; boundary=b",
+                MultiPartRepresentation.setBoundary(
+                                MediaType.valueOf(
+                                        "multipart/mixed; boundary=a; deferSpec=20220824"),
+                                "b")
+                        .toString());
+        assertEquals(
+                "multipart/related; type=\"a;boundary=x\"; boundary=b",
+                MultiPartRepresentation.setBoundary(
+                                MediaType.valueOf(
+                                        "multipart/related; Boundary=a; type=\"a;boundary=x\""),
+                                "b")
+                        .toString());
+        // As parsed from a Content-Type header
+        Series<Parameter> parameters = new Series<>(Parameter.class);
+        parameters.add("type", "application/xml");
+        parameters.add("BOUNDARY", "a");
+        assertEquals(
+                "multipart/related; type=\"application/xml\"; boundary=b",
+                MultiPartRepresentation.setBoundary(
+                                new MediaType("multipart/related", parameters), "b")
+                        .toString());
+        assertNull(MultiPartRepresentation.setBoundary(null, "b"));
+        assertEquals(
+                MediaType.MULTIPART_FORM_DATA.getDescription(),
+                MultiPartRepresentation.setBoundary(MediaType.MULTIPART_FORM_DATA, "b")
+                        .getDescription());
+        assertEquals(
+                "b",
+                MultiPartRepresentation.getBoundary(
+                        MultiPartRepresentation.setBoundary(new MediaType((String) null), "b")));
+
+        MultiPartRepresentation rep =
+                new MultiPartRepresentation(
+                        MediaType.valueOf("multipart/mixed; deferSpec=20220824"),
+                        "b",
+                        List.of(new Part("a", null, new StringRepresentation("b"))));
+        assertEquals(
+                "multipart/mixed; deferSpec=20220824; boundary=b", rep.getMediaType().toString());
+        rep.setBoundary("c");
+        assertEquals(
+                "multipart/mixed; deferSpec=20220824; boundary=c", rep.getMediaType().toString());
+    }
+
     @Test
     void testRandomBoundary() {
         MultiPartRepresentation rep =