Saitō was born on April 22, 1991 in Shōwa, Yamanashi Prefecture, Japan. He's the eldest of three siblings, with two younger sisters. He started wanting to become a voice actor in high school, inspired by Akira Ishida's role as Koyemshi in Bokurano. Prior to that, he wanted to become a musician or a novel writer.
Saitō joined the second 81 Produce’s Audition in 2008. In that audition, a total of 1,035 people participated with only 33 went to the final audition to decide who would win and stay training under the talent agency. He won the audition for the male category and started taking voice acting classes at 81 ACTOR'S STUDIO afterwards, while still attending high school. In 2009, Saitō won the "Judges' Special Prize" in the recitation competition at the 33rd National High School Comprehensive Cultural Festival. Veteran voice actor Nobuyo Ōyama was one of the judges present at that time. He started his career as a voice actor in 2010, voicing minor roles in few anime and video games. He put less focus on his career as a voice actor during his first two years to focus himself on his studies at Waseda University. After graduating from college in 2014, he started to take on more important roles. The first major role that he got through audition was Tasuku Ryuenji in the anime seriesFuture Card Buddyfight. In the same year, Saitō got other major roles as Tadashi Yamaguchi in the anime series Haikyu!!, Tatsumi in the anime series Akame ga Kill!, and Twelve in the anime series Terror in Resonance. At the 9th Seiyu Awards in 2015, Saitō won the "Best Male Newcomer" award for his role as Twelve in the anime series Terror in Resonance. He shared the award with Ryōta Ōsaka and Natsuki Hanae. In March 2019, he and the other members of received a singing award in the 13thVoice Actor Awards.
Saitō made his debut as a solo artist in 2017, under Sacra Music. He released his first single "Fish Story" on June 7, 2017. His second single "Yoake wa Mada/Hikari Tatsu Ame" was released on September 6, 2017. The song "Hikari Tatsu Ame" was used as the opening theme song to Katsugeki/Touken Ranbu. Saitō released his third single "Date" on June 20, 2018. For his third single, he took upon himself songwriting duties, writing lyrics and music for all songs featured on it. Saitō released his first album, quantum stranger, on December 19, 2018. On February 24, 2019, Saitō held his 1st solo live concert entitled "Saitō Sōma 1st LIVE quantum stranger" in Maihama Amphi Theater. A blu-ray edition of the concert was released on June 5 of the same year the concert was held.