In the first round of the civil war between grandfather and grandson, Andronikos III was successful in getting recognition for himself as junior emperor, with Thrace as his personal appanage. Throughout the conflict, Synadenos was among the most radical and uncompromising supporters of the younger Andronikos, favouring the eventual deposition of Andronikos II. During the course of the war, Synadenos defeated loyalist forces under Constantine Asen, and in the final round of the civil war, in 1327–28, he served as governor in Thrace. As more and more localities, including the Empire's second-most important city, Thessalonica, switched over to Andronikos III's camp, Synadenos along with Kantakouzenos urged Andronikos III to march on Constantinople and depose his grandfather outright. On the night of 13 May 1328, the three men led their army through a gate after bribing its guard, taking possession of the imperial capital without resistance. Andronikos II was forced to resign the throne and retire to a monastery, with his grandson succeeding him as sole emperor. For his services, Synadenos was promoted to "protostrator" sometime during the civil war, perhaps as early as 1321.