The Siege of Ariminum, also known as the Siege of Rimini, was an encounter in 538 AD during the Gothic War, where the Byzantine forces broke the siege by the Ostrogoths (Goths).
In early 538 and during the Siege of Rome (537–538), the besieged Byzantine General Belisarius sent John the Sanguinary to raid the Picenum region. John took the initiative and captured Ariminum, near the Gothic capital, Ravenna, to entice the Goths into lifting the Siege of Rome. Belisarius sent reinforcements to Ariminum and ordered John to leave the city, as his cavalry forces would not be useful in the case of a siege. However, John refused to obey and stayed in the city. In the meantime, fearing for their capital, the Goths retreated from Rome in mid-March 538 and moved north. In April they besieged Ariminum, trapping John and his troops. They were unsuccessful in taking Ariminum by force, and instead, the leader of the Gothic forces, Vitiges, decided to starve the town.
Initially, the Byzantine leadership was divided over whether to rescue John despite his insubordination. Belisarius ultimately chose to save him after an appeal from Chamberlain Narses that John's loss would boost the morale of the Goths. Belisarius wanted to avoid a direct engagement because the Goths outnumbered the Byzantine forces. He split his army into smaller groups and sent them to Ariminum by land and sea. The near-simultaneous arrival of Byzantine forces from multiple directions created the impression of a much larger army, which unnerved the Goths. As a result, they abandoned the siege in late summer 538 and withdrew to Ravenna. Despite the victory, disagreements over John's insubordination fractured the unity of the Byzantine leadership and shaped the course of the Gothic War.