diff --git a/src/moov/trak/mdia/minf/stbl/stsd/mod.rs b/src/moov/trak/mdia/minf/stbl/stsd/mod.rs
index f7144f4f..01f85ea8 100644
--- a/src/moov/trak/mdia/minf/stbl/stsd/mod.rs
+++ b/src/moov/trak/mdia/minf/stbl/stsd/mod.rs
@@ -125,7 +125,7 @@ pub enum Codec {
     Samr(Samr),
 
     // Unknown
-    Unknown(FourCC),
+    Unknown(FourCC, Vec<u8>),
 }
 
 impl Decode for Codec {
@@ -157,9 +157,23 @@ impl Decode for Codec {
             Any::S16l(atom) => atom.into(),
             Any::Wvtt(atom) => atom.into(),
             Any::Samr(atom) => atom.into(),
+            unknown @ Any::Unknown(..) => {
+                crate::decode_unknown(&unknown, Stsd::KIND)?;
+
+                let Any::Unknown(kind, body) = unknown else {
+                    unreachable!()
+                };
+                Self::Unknown(kind, body)
+            }
             unknown => {
                 crate::decode_unknown(&unknown, Stsd::KIND)?;
-                Self::Unknown(unknown.kind())
+
+                // The atom kind is known elsewhere in the hierarchy, but is not a supported
+                // sample entry. Re-encode its body so the unknown entry remains a valid box.
+                let kind = unknown.kind();
+                let mut encoded = Vec::new();
+                unknown.encode(&mut encoded)?;
+                Self::Unknown(kind, encoded.split_off(8))
             }
         })
     }
@@ -168,7 +182,19 @@ impl Decode for Codec {
 impl Encode for Codec {
     fn encode<B: BufMut>(&self, buf: &mut B) -> Result<()> {
         match self {
-            Self::Unknown(kind) => kind.encode(buf),
+            Self::Unknown(kind, body) => {
+                let start = buf.len();
+                0u32.encode(buf)?;
+                kind.encode(buf)?;
+                body.encode(buf)?;
+
+                let size: u32 = (buf.len() - start)
+                    .try_into()
+                    .map_err(|_| Error::TooLarge(*kind))?;
+                buf.set_slice(start, &size.to_be_bytes());
+
+                Ok(())
+            }
             Self::Avc1(atom) => atom.encode(buf),
             Self::Hev1(atom) => atom.encode(buf),
             Self::Hvc1(atom) => atom.encode(buf),
diff --git a/tests/stsd.rs b/tests/stsd.rs
new file mode 100644
index 00000000..51aac39c
--- /dev/null
+++ b/tests/stsd.rs
@@ -0,0 +1,20 @@
+#![cfg(not(feature = "strict"))]
+
+use mp4_atom::{Codec, Decode, Encode, FourCC, Stsd};
+
+#[test]
+fn unknown_codec_round_trips() {
+    let input = b"\0\0\0\x1cstsd\0\0\0\0\0\0\0\x01\0\0\0\x0cdvh1\x01\x02\x03\x04";
+    let mut buf = input.as_slice();
+
+    let stsd = Stsd::decode(&mut buf).unwrap();
+    assert_eq!(
+        stsd.codecs,
+        vec![Codec::Unknown(FourCC::new(b"dvh1"), vec![1, 2, 3, 4])]
+    );
+
+    let mut output = Vec::new();
+    stsd.encode(&mut output).unwrap();
+
+    assert_eq!(output, input);
+}