Middle and Late Cambrian reefs were built mainly by cyanobacterial communities. A few reefs with a metazoan as well as an algal component, however, are known from this interval. A Middle Cambrian reef formed primarily by spicular demosponges is described here from the Mila Formation in the Elburz Mountains, northern Iran. The reef is enclosed within calcareous grainstones which contain terminal Middle Cambrian (late Mayan) trilobites. The Mila Formation reef was constructed by sponges of the family Anthaspidellidae and bacterial (algal?) sheaths, and is the earliest metazoan reef to be documented from the interval after the demise of archaeocyath sponges. The reefal community is typical of subsequent reefal communities of Early–Middle Ordovician age. The Ordovician examples differ only by the incorporation of additional metazoan elements.