Tahmasp I was the second Shah of Safavid Iran from 1524 to 1576. He was the eldest son of Ismail I and his principal consort, Tajlu Khanum. Ascending the throne after the death of his father on 23 May 1524, the first years of Tahmasp's reign were marked by civil wars between the Qizilbash leaders until 1532, when he asserted his authority and began an absolute monarchy. He soon faced a longstanding war with the Ottoman Empire, which was divided into three phases. The Ottomans, under Suleiman the Magnificent, tried to put their favoured candidates on the Safavid throne. The war ended with the Peace of Amasya in 1555, with the Ottomans gaining sovereignty over Baghdad, much of Kurdistan and western Georgia. Tahmasp also had conflicts with the Uzbeks of Bukhara over Khorasan, with them repeatedly raiding Herat. He led an army in 1528 (when he was fourteen), and defeated the Uzbeks in the Battle of Jam; he used artillery, unknown to the other side.

Tahmasp was a patron of the arts, building a royal house of arts for painters, calligraphers and poets, and was an accomplished painter himself. Later in his reign he came to despise poets, shunning many and exiling them to India and the Mughal court. Tahmasp is known for his religious piety and fervent zealotry for the Sh'ia branch of Islam. He bestowed many privileges on the clergy and allowed them to participate in legal and administrative matters. In 1544 he demanded that the fugitive Mughal emperor Humayun convert to Shi'ism in return for military assistance to reclaim his throne in India. Nevertheless, Tahmasp still negotiated alliances with the Christian powers of the Republic of Venice and the Habsburg monarchy.

Tahmasp's reign of nearly fifty-two years was the longest of any member of the Safavid dynasty. Although contemporary Western accounts were critical, modern historians describe him as a courageous and able commander who maintained and expanded his father's empire. His reign saw a shift in the Safavid ideological policy; he ended the worshipping of his father as the Messiah by the Turkoman Qizilbash tribes and instead established a public image of a pious and orthodox Sh'ia king. He started a long process followed by his successors to end the Qizilbash influence on Safavid politics, replacing them with the newly-introduced 'third force' containing Islamised Georgians and Armenians.