diff --git a/spring-web/src/main/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriter.java b/spring-web/src/main/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriter.java
index 63c0f9bfd40..8f024f7602f 100644
--- a/spring-web/src/main/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriter.java
+++ b/spring-web/src/main/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriter.java
@@ -269,13 +269,18 @@ public class MultipartHttpMessageWriter implements HttpMessageWriter<MultiValueM
 		Publisher<T> bodyPublisher =
 				body instanceof Publisher ? (Publisher<T>) body : Mono.just(body);
 
-		Mono<Void> partWritten = ((HttpMessageWriter<T>) writer.get())
+		// The writer will call MultipartHttpOutputMessage#write which doesn't actually write
+		// but only stores the body Flux and returns Mono.empty().
+
+		Mono<Void> partContentReady = ((HttpMessageWriter<T>) writer.get())
 				.write(bodyPublisher, resolvableType, contentType, outputMessage, Collections.emptyMap());
 
-		return Flux.concat(
-				Mono.just(generateBoundaryLine(boundary)),
-				partWritten.thenMany(Flux.defer(outputMessage::getBody)),
-				Mono.just(generateNewLine()));
+		// After partContentReady, we can access the part content from MultipartHttpOutputMessage
+		// and use it for writing to the actual request body
+
+		Flux<DataBuffer> partContent = partContentReady.thenMany(Flux.defer(outputMessage::getBody));
+
+		return Flux.concat(Mono.just(generateBoundaryLine(boundary)), partContent, Mono.just(generateNewLine()));
 	}
 
 
@@ -353,7 +358,9 @@ public class MultipartHttpMessageWriter implements HttpMessageWriter<MultiValueM
 				return Mono.error(new IllegalStateException("Multiple calls to writeWith() not supported"));
 			}
 			this.body = Flux.just(generateHeaders()).concatWith(body);
-			return this.body.then();
+
+			// We don't actually want to write (just save the body Flux)
+			return Mono.empty();
 		}
 
 		private DataBuffer generateHeaders() {
@@ -387,8 +394,7 @@ public class MultipartHttpMessageWriter implements HttpMessageWriter<MultiValueM
 
 		@Override
 		public Mono<Void> setComplete() {
-			return (this.body != null ? this.body.then() :
-					Mono.error(new IllegalStateException("Body has not been written yet")));
+			return Mono.error(new UnsupportedOperationException());
 		}
 	}
 
diff --git a/spring-web/src/test/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriterTests.java b/spring-web/src/test/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriterTests.java
index 1d42d34d6f3..81594d3e2d9 100644
--- a/spring-web/src/test/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriterTests.java
+++ b/spring-web/src/test/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriterTests.java
@@ -16,6 +16,7 @@
 
 package org.springframework.http.codec.multipart;
 
+import java.io.IOException;
 import java.time.Duration;
 import java.util.Collections;
 import java.util.List;
@@ -25,6 +26,7 @@ import org.junit.Test;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
+import reactor.core.publisher.UnicastProcessor;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.StringDecoder;
@@ -146,9 +148,48 @@ public class MultipartHttpMessageWriterTests {
 				Collections.emptyMap()).block(Duration.ZERO);
 
 		assertEquals("foobarbaz", value);
+	}
+
+	@Test // SPR-16402
+	public void singleSubscriberWithResource() throws IOException {
+		UnicastProcessor<Resource> processor = UnicastProcessor.create();
+		Resource logo = new ClassPathResource("/org/springframework/http/converter/logo.jpg");
+		Mono.just(logo).subscribe(processor);
+
+		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
+		bodyBuilder.asyncPart("logo", processor, Resource.class);
+
+		Mono<MultiValueMap<String, HttpEntity<?>>> result = Mono.just(bodyBuilder.build());
+
+		MockServerHttpResponse response = new MockServerHttpResponse();
+		Map<String, Object> hints = Collections.emptyMap();
+		this.writer.write(result, null, MediaType.MULTIPART_FORM_DATA, response, hints).block();
+
+		MultiValueMap<String, Part> requestParts = parse(response, hints);
+		assertEquals(1, requestParts.size());
 
+		Part part = requestParts.getFirst("logo");
+		assertEquals("logo", part.name());
+// TODO: a Resource written as an async part doesn't have a file name in the contentDisposition
+//		assertTrue(part instanceof FilePart);
+//		assertEquals("logo.jpg", ((FilePart) part).filename());
+		assertEquals(MediaType.IMAGE_JPEG, part.headers().getContentType());
+		assertEquals(logo.getFile().length(), part.headers().getContentLength());
+	}
 
+	@Test // SPR-16402
+	public void singleSubscriberWithStrings() {
+		UnicastProcessor<String> processor = UnicastProcessor.create();
+		Flux.just("foo", "bar", "baz").subscribe(processor);
 
+		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
+		bodyBuilder.asyncPart("name", processor, String.class);
+
+		Mono<MultiValueMap<String, HttpEntity<?>>> result = Mono.just(bodyBuilder.build());
+
+		MockServerHttpResponse response = new MockServerHttpResponse();
+		Map<String, Object> hints = Collections.emptyMap();
+		this.writer.write(result, null, MediaType.MULTIPART_FORM_DATA, response, hints).block();
 	}
 
 	private MultiValueMap<String, Part> parse(MockServerHttpResponse response, Map<String, Object> hints) {