We report one of the earliest known circular plazas in Andean South America and one of the earliest examples of monumental, megalithic ceremonial architecture in the Americas. The example presented here is constructed of large free-standing and vertically placed megalithic stones and is located in the Cajamarca basin of the northern Peruvian Andes. This construction method has never before been reported in the Andes and is distinct from other monumental circular plazas in the region. We present three radiocarbon dates associated with the initial construction of the plaza which average to approximately 2750 calibrated years before the common era (cal BCE), corresponding squarely to the Late Preceramic period, which saw the first monumental construction in the Andes. This is one of the earliest examples of monumental, megalithic architecture in the Americas.