Financial details were not disclosed but Scottish media reported the transfer fee was £25 million ($31.73 million).
Al-Ittihad announced the deal by publishing a video of coach Nuno Espirito Santo watching Jota's goals.
"We want to conquer the world," the former Wolverhampton Wanderers coach said in the video with the FIFA Club World Cup in the background.
Al-Ittihad will participate in the Club World Cup, which will be held in Jeddah in December, as champions of the host country's league.
The former Benfica forward scored 28 goals and made 26 assists in 83 games after moving to the Scottish champions in 2021.
The Saudi Pro league has become an attractive destination for Europe-based players since Cristiano Ronaldo moved to Al-Nassr last January.
Last month, the Saudi Public Investment Fund (PIF) announced a Sports Clubs Investment and Privatization Project involving Al-Ittihad, Al-Ahli, Al-Nassr and Al-Hilal.
PIF owns 75% of each of the four clubs, while their respective non-profit foundations own 25% of each.