Pachydiadema is an extinct genus of acorn barnacles in the order Balanomorpha that lived in what is now Sweden during the early Campanian. The genus is monospecific, containing a single species, Pachydiadema cretaceum.
It is the only genus in the family Pachydiadematidae and superfamily Pachydiadematoidea, both of which are monotypic. It was originally described as a subgenus of Catophragmus, but was redescribed as a separate genus in 2014.