diff --git a/libavformat/mpegtsenc.c b/libavformat/mpegtsenc.c
index 00ad426086..a4a2d8cdaf 100644
--- a/libavformat/mpegtsenc.c
+++ b/libavformat/mpegtsenc.c
@@ -1835,6 +1835,13 @@ static int opus_get_packet_samples(AVFormatContext *s, AVPacket *pkt)
     return duration;
 }
 
+// Copies `size_bytes` from `source` to `dest`.
+// Returns a pointer to the next destination address after the range written.
+static uint8_t* append(uint8_t* dest, const uint8_t* source, int size_bytes) {
+  memcpy(dest, source, size_bytes);
+  return dest + size_bytes;
+}
+
 static int mpegts_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
 {
     AVStream *st = s->streams[pkt->stream_index];
@@ -1877,6 +1884,7 @@ static int mpegts_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
 
     if (st->codecpar->codec_id == AV_CODEC_ID_H264) {
         const uint8_t *p = buf, *buf_end = p + size;
+        const uint8_t *found_aud = NULL, *found_aud_end = NULL;
         uint32_t state = -1;
         int extradd = (pkt->flags & AV_PKT_FLAG_KEY) ? st->codecpar->extradata_size : 0;
         int ret = ff_check_h264_startcode(s, st, pkt);
@@ -1886,17 +1894,34 @@ static int mpegts_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
         if (extradd && AV_RB24(st->codecpar->extradata) > 1)
             extradd = 0;
 
+        // Ensure that all pictures are prefixed with an AUD, and that
+        // IDR pictures are also prefixed with SPS and PPS. SPS and PPS
+        // are assumed to be available in 'extradata' if not found in-band.
         do {
             p = avpriv_find_start_code(p, buf_end, &state);
             av_log(s, AV_LOG_TRACE, "nal %"PRId32"\n", state & 0x1f);
-            if ((state & 0x1f) == 7)
+            if ((state & 0x1f) == 7)  // SPS NAL
                 extradd = 0;
-        } while (p < buf_end && (state & 0x1f) != 9 &&
-                 (state & 0x1f) != 5 && (state & 0x1f) != 1);
-
-        if ((state & 0x1f) != 5)
+            if ((state & 0x1f) == 9) {  // AUD NAL
+                found_aud = p - 4;  // start of the 0x000001 start code.
+                found_aud_end = p + 1; // first byte past the AUD.
+
+                if (found_aud < buf)
+                    found_aud = buf;
+                if (buf_end < found_aud_end)
+                    found_aud_end = buf_end;
+            }
+        } while (p < buf_end
+                 && (state & 0x1f) != 5  // IDR picture
+                 && (state & 0x1f) != 1  // non-IDR picture
+                 && (extradd > 0 || !found_aud));
+        if ((state & 0x1f) != 5) {
+            // Did not find an IDR picture; do not emit extradata.
             extradd = 0;
-        if ((state & 0x1f) != 9) { // AUD NAL
+        }
+
+        if (!found_aud) {
+            // Prefix 'buf' with the missing AUD, and extradata if needed.
             data = av_malloc(pkt->size + 6 + extradd);
             if (!data)
                 return AVERROR(ENOMEM);
@@ -1907,6 +1932,24 @@ static int mpegts_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
             data[5] = 0xf0; // any slice type (0xe) + rbsp stop one bit
             buf     = data;
             size    = pkt->size + 6 + extradd;
+        } else if (extradd != 0) {
+            // Move the AUD up to the beginning of the frame, where the H.264
+            // spec requires it to appear. Emit the extradata after it.
+            const int aud_size = found_aud_end - found_aud;
+            const int new_pkt_size = pkt->size + 1 + extradd;
+            uint8_t *pos;
+            data = av_malloc(new_pkt_size);
+            if (!data)
+                return AVERROR(ENOMEM);
+            AV_WB8(data, 0x0);
+            pos = data + 1;
+            pos = append(pos, found_aud, aud_size);
+            pos = append(pos, st->codecpar->extradata, extradd);
+            pos = append(pos, pkt->data, found_aud - pkt->data);
+            pos = append(pos, found_aud_end, buf_end - found_aud_end);
+            av_assert0(data + new_pkt_size == pos);
+            buf     = data;
+            size    = new_pkt_size;
         }
     } else if (st->codecpar->codec_id == AV_CODEC_ID_AAC) {
         if (pkt->size < 2) {
