The aim of this article is to present the first results of a project investigating the hitherto unexplored site of Pergamos, located on a hill known as Koules/Alonaki just to the south of modern Moustheni in the Pieria valley. The site has a long history, with the earliest traces of human activity dating to the Late Bronze Age and the first visible architectural remains—impressive and well-preserved fortifications built in a stack-work technique—originating in the Late Archaic period. Following this, surface finds testify to activity and habitation throughout the centuries until a substantial strengthening of the defensive walls during Late Roman or medieval times, before the site was abandoned at a currently unknown point. To achieve the aim the article begins with an introduction to the site and its surrounding landscape. Following this, the results of a survey of the visible remains are presented, focusing on the fortifications and two architectural fragments. The site is then discussed within its spatial and chronological context, suggesting that Pergamos was an important settlement in the area, exhibiting traits of a polis, in particular during Archaic and Classical times before it fell within the sphere of influence of the Roman colony Philippi.