A native of Miyagi Prefecture, Saitō attended military preparatory schools and graduated from the 24th class of the Imperial Japanese Army Academy in 1912 as a cavalryman.

As Japan had lost control of the sea lanes by this time in the Pacific War, the division suffered heavy casualties due to submarine attacks on its transports during the move.

Admiral Chuichi Nagumo, commander of the Central Pacific Area Fleet was also on the island, and was consulted frequently by Saitō.

Without possibility of resupply, the situation was hopeless for the defenders, but Saito was determined to fight to the last man, as he was aware that the fall of Saipan would place the Japanese archipelago within range of American strategic bombers.

Saitō, wanting to avoid capture and dishonor, committed seppuku in a cave at dawn on 10 July, with his adjutant shooting him in the head after he had disemboweled himself.