Lawmakers of the ruling Liberal Democratic Party and its junior coalition partner, the Komieto party, voted to elect Ishiba after he won an intra-party leadership race last week, Anadolu Agency reported.
He secured the support of 291 lawmakers out of 461 ballots cast in the lower house of the parliament.
Ishiba, 67, succeeds Fumio Kishida, who resigned early Tuesday, along with his Cabinet.
Kishida has served as premier since October 2021 after he succeeded former Prime Minister Suga Yoshihide.
The newly-elected prime minister had earlier served as defense minister and was elected LDP president on his fifth attempt.
He has called for snap elections later this month on Oct. 27, the shortest period by any Japanese LDP leader after they were elected as party presidents.
SD/