Macroalgae play a crucial ecological and biogeochemical role in modern marine ecosystems. Although eukaryotic algae have a deep history within the Precambrian, most pre-Cryogenian alga fossils are microscopic and macroalga fossils are poorly documented during this period. Here we report a well-preserved assemblage of macroscopic and diversified carbonaceous compression fossils from the Tonian Shiwangzhuang Formation in western Shandong, North China. This assemblage consists of seventeen distinct morphotaxa, including eleven macroalgae, showing remarkable taxonomic diversity that is more diverse than many other Tonian macrofossil assemblages all over the world. Three taxa from the Shiwangzhuang assemblage show evidence of simple multicellularity with two of them consisting of giant cells and comparable to giant-celled green algae, and seven taxa with diagnostic transverse annulations are likely coenocytic algae. The Shiwangzhuang assemblage indicates that some eukaryotic clades had achieved macroscopic growth through simple multicellularity or coenocytism, paving the way, either ecologically or phylogenetically, for the eventual appearance of complex multicellularity. In addition, there are some biostratigraphically significant genera, including the Chuaria-Tawuia and Sinosabellidites-Protoarenicola-Pararenicola assemblages, indicating a Tonian age for the Shiwangzhuang Formation. This study also suggests that the Tonian diversity of multicellular or coenocytic macroalgae is probably underestimated, pending for a systematic and thorough investigation of Tonian macroscopic carbonaceous compressions. Macroalgae may have an initial ecological expansion, at least locally if not globally, in the Tonian Period.