By the accession of Andronicus in 1328, the Imperial territories in Anatolia had dramatically shrunk from almost all of the west of modern Turkey forty years earlier to a few scattered outposts along the Aegean Sea and a small core province around Nicomedia within about 150 km of the capital city Constantinople. Recently the Ottoman Turks had captured the important city of Prusa (Bursa) in Bithynia. Andronicus decided to relieve the important besieged cities of Nicomedia and Nicaea, and hoped to restore the frontier to a stable position.

Andronicus led an army of about 4,000 men, which was the greatest he could muster. They marched along the Sea of Marmara towards Nicomedia. At Pelekanon, a Turkish army led by Orhan I had encamped on the hills to gain a strategic advantage and blocked the road to Nicomedia. On 10 June, Orhan sent 300 cavalry archers downhill to lure the Byzantines unto the hills, but these were driven off by the Byzantines, who were unwilling to advance further. The belligerent armies engaged in indecisive clashes until nightfall. The Byzantine army prepared to retreat, but the Turks gave them no chance. Both Andronicus and Cantacuzene were lightly wounded, while rumors spread that the Emperor had either been killed or mortally wounded, resulting in panic. Eventually the retreat turned into a rout with heavy casualties on the Byzantine side. Cantacuzene led the remaining Byzantine soldiers back to Constantinople by sea.