Tropaeum is an extinct genus of ammonites found throughout the oceans of the world during the Early Cretaceous.

As with many other members of the family Ancyloceratidae, there was a trend among species within this genus to uncoil somewhat, in a very similar manner to the genus Lytoceras.

The largest species, T. imperator of Australia, had a shell a little over one meter in diameter.

The name "Tropaeum" was applied by paleontologist James De Carle Sowerby, in 1837.

This ammonite-related article is a stub.