diff --git a/fitsio/src/threadsafe_fitsfile.rs b/fitsio/src/threadsafe_fitsfile.rs
index a4b7262b..d002ba4c 100644
--- a/fitsio/src/threadsafe_fitsfile.rs
+++ b/fitsio/src/threadsafe_fitsfile.rs
@@ -24,6 +24,8 @@ pub struct ThreadsafeFitsFile(Arc<Mutex<FitsFile>>);
 // Safety: we explicitly wrap the type in an Arc::Mutex which is threadsafe. The Mutex ensures that
 // only one thread can be modifying the file at once.
 unsafe impl Send for ThreadsafeFitsFile {}
+// Safety: the inner Mutex ensures exclusive access when shared across threads via &ThreadsafeFitsFile.
+unsafe impl Sync for ThreadsafeFitsFile {}
 
 impl FitsFile {
     /**