What was the first capital of the Muslim world?

Medina was the initial capital of the Muslim world. Following the migration (Hijra) of the Prophet Muhammad and his companions from Mecca to Medina in 622 CE, Medina became the focal point of the early Muslim community and the seat of the first Islamic state. It was the capital for the early Muslim Caliphate under the authority of the Prophet Muhammad and the first four caliphs, referred to as the Rashidun Caliphs.

But soon after the Prophet Muhammad's death, the capital was finally transferred to Kufa under Caliph Ali (fourth caliph) later. Subsequently, the capital of the Islamic world came to be in Damascus under the Umayyad Caliphate (661-750 CE) and later in Baghdad under the Abbasid Caliphate (750-1258 CE).

Therefore, although Medina was the initial capital, the form of the capital of the Islamic empire changed through time as political and military configurations changed.