In the summer of the same year, the Byzantines gathered an army and without a declaration of war advanced towards Bulgaria, looting and plundering the villages on their way. The Emperor faced the Bulgarians at the village of Rusokastro. 

Ivan Alexander had troops of 8,000 while the Byzantines were only 3,000. There were negotiations between the two rulers but the Bulgarian emperor deliberately prolonged them because he was awaiting reinforcements. In the night of July 17 they finally arrived in his camp (3,000 cavalrymen) and he decided to attack the Byzantines the next day. Andronikos III Palaiologos had no choice but to accept the fight.

The battle began at six in the morning and continued for three hours. The Byzantines tried to prevent the Bulgarian cavalry from surrounding them, but their manoeuvre failed. The cavalry moved around the first Byzantine line, leaving it for the infantry and charged the rear of their flanks. After a fierce fight the Byzantines were defeated, abandoned the battlefield and took refuge in Rusokastro. The Bulgarian army surrounded the fortress and at noon on the same day Ivan Alexander sent envoys to continue the negotiations. The Bulgarians recovered their lost territory in Thrace and strengthened the position of their empire. This was the last major battle between Bulgaria and Byzantium as their seven-century rivalry for domination of the Balkans was soon to come to an end, after the fall of the two Empires under Ottoman domination.