Haeckelites are members of a proposed family of hypothetical carbon allotropes. The carbon atoms would be arranged in a trivalently coordinated structure generated by a periodic arrangement of pentagonal, hexagonal and heptagonal carbon rings. They have not yet been synthesised in the laboratory, but have been the subject of a considerable amount of theoretical work and numerical simulation. They were first proposed by Humberto and Mauricio Terrones and their colleagues in 2000.

They were named in honour of Ernst Haeckel, whose diagrams of Radiolaria and Phaeodarea contained similar structural features.