Callithamnion is a genus of algae belonging to the family Callithamniaceae.

The genus was first described by Danish botanist Hans Christian Lyngbye in 1819, and the type species is Callithamnion corymbosum (Smith) Lyngbye.,

The genus has cosmopolitan distribution. Species are found in Europe (including Norway and Great Britain,), Australia, America (including Massachusetts, New York, North Carolina and Georgia), Newfoundland (Canada), Sri Lanka and South Africa.

The genus of Callithamnion has undergone 2 major changes in its history. Carl Nägeli (in 1861) transferred species without alternate branchlets to Antithamnion, Rhodochorton and Acrochaetium. Then Genevieve Feldmann-Mazoyer in 1941 created genus Aglaothamnion for species having uninucleate cells, zig-zag carpogonial branches and lobed groups of carposporangia, and re-circumscribed Callithamnion. Aglaothamnion is now sometimes regarded as a synonym of Callithamnion with insufficient evidence for separate evolutionary lines of development.

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