Grammoceras is an extinct genus of ammonite found in Yorks Ravenscar, England, from Jurassic period sediments.

Its overall distribution is fairly worldwide.

Grammoceras has a thin, discoidally evolute shell with simple, gently sigmoid ribbing and a low ventral keel.

Dumortiera and Catulloceras from the same age are similar except that Dumortiera has a more rounded venter and Catulloceras has a subquadrate whorl section.

Asthenoceras from the slightly younger lower Bajocian may be a subgenus.