Cambrian Burgess Shale-type fossil Lagerst & auml;tten yield abundant and diverse animal fossils preserved with both skeletonized and soft tissues, and hence provide more complete evidence for understanding the rapid diversification of animal phyla in the early Cambrian. However, macroalgae have received less attention despite their abundance and ecological importance for animal evolution. Notably, macroalgae preserved with cellular features have not been recognized in Cambrian fossil Lagerst & auml;tten, which hinders the interpretation of their affinities. Here, we report an unbranched, multicellular and filamentous macroalga Chaetomorphium cambria gen. et sp. nov. from the Chengjiang biota, representing the first reported macroalgal species preserved with cellular features from the Cambrian Burgess Shale-type fossil Lagerst & auml;tten. Chaetomorphium is characterized by uniseriate unbranched filaments composed of relatively large cells, filament growth through intercalary cell division, and asexual reproduction through akinetes. Accordingly, Chaetomorphium is assigned to the total-group of the Cladophorales in the class Ulvophyceae of the phylum Chlorophyta. The new finding demonstrates that early unbranched filamentous cladophoraleans acquired cellular differentiation, and intercalary cell division no later than the early Cambrian.