Seydi Ali Reis (1498–1563), formerly also written Sidi Ali Reis and Sidi Ali Ben Hossein, was an Ottoman admiral and navigator. Known also as Katib-i Rumi, Galatalı or Sidi Ali Çelebi, he commanded the left wing of the Ottoman fleet at the naval Battle of Preveza in 1538. He was later promoted to the rank of fleet admiral of the Ottoman fleet in the Indian Ocean, and as such, encountered the Portuguese forces based in the Indian city of Goa on several occasions in 1554. Seydi was able to unite several Muslim countries on the coast of the Arabian Sea against the Portuguese.

The Battle of Preveza was a naval engagement that took place on 28 September 1538 near Preveza in the Ionian Sea in northwestern Greece between an Ottoman fleet and that of a Holy League. The battle was an Ottoman victory which occurred in the same area in the Ionian Sea as the Battle of Actium in 31 BC. It was one of the three largest sea battles that took place in the sixteenth century Mediterranean, along with the Battle of Djerba and the Battle of Lepanto.