Diplomoceras is a genus of ammonites included in the family Diplomoceratidae.

[2] Fossils of species within this genus have been found in the Late Cretaceous sediments (age range: Campanian-Maastrichtian).

[3][4] D. maximum had coiled shell length about 1.5 m (4.9 ft), uncoiled shell being 3 m (9.8 ft) to over 4 m (13 ft), with body chamber around 2 m (6.6 ft).

[5][6][7] Some partial specimens may belong to shell with length around 1.7 m (5.6 ft).

[8] Studies of Diplomoceras suggest that members of this genus could reach lifespans of over 200 years.