This study describes archaeological excavations carried out at Unguja Ukuu on the main island of Zanzibar, Tanzania. The site has long remained obscure, oral histories do not mention it and no particular group among the living community of the island describes its origin from the site. A stone well at Unguja Ukuu together with several other early monuments of the east African coast that survive on the site have been attributed to the Wadebuli, suspected by early scholars to be people of Arab descent from their colonies in India or elsewhere on the Islands of eastern Indian Ocean. Surface survey and the drilling of more than 200 cores have defined the lateral extent and the stratigraphy of the site. Unguja Ukuu is a large site (c.16–17 ha) and the study reveals that it is a major center of an African iron-using farming community who occupied it from c. 500 AD. Radiocarbon dating and pottery provide the basis for this chronology. The study addresses an old controversy whether some of the pre-stone built settlements that developed on the east African coast could be indications of urbanization. Knowledge of the functional specialization of the settlement prior to its abandonment c. 900 AD is based on the evidence on the density of craft activity, community engagement in the regional trade with the mainland African continent, as far away as Roman Egypt, and in the interregional trade connected to the Indian Ocean, as well as redistribution of foreign merchandise to other sites and areas in the region. These as well as the location of the site linking the external trade and the mainland resource base indicate that Unguja Ukuu was a key urban centre built of mud and timber structures. This challenges our previous understanding of 8–9th centuries AD as the onset of early urbanism on the east African coast. The study proposes cycles of urbanism and emphasizes the need to reassess the problem of early urban identity and the use of wide range of criteria to overcome limitations of previous early urban investigations south of the Sahara and beyond. The results of the investigation given in this study are relevant to the history and archaeology of Zanzibar and the rest of East Africa and make a contribution particularly to extending the known time depth of the early urban tradition often conceived to occur in the late first millennium ad.