It brings to a close a pursuit that had attracted competition from Newcastle United, Chelsea, Tottenham Hotspur and a host of other European clubs for the 18-year-old Nigerian.
The fee remains undisclosed. Yohanna will complete the formalities of his move when the transfer window officially opens, subject to the standard regulatory approvals. He turns 19 later this month.
Manager Fabian Hürzeler spoke warmly about the signing.
"I'm looking forward to working with Zadok. Having seen his games and his attributes, he is a player that can impact games in the final third. He's still young, and will need time to adapt to the club and Premier League, but he's an exciting player to watch and he brings the kind of creativity we know our fans will enjoy. He's dynamic, has pace and likes to take players on. His attributes and ability will be a real addition to our attacking options."
Yohanna chose Brighton over rival suitors — and it was the club's philosophy around young player development that proved decisive.
"I watch Premier League games a lot, so I know the teams that use young players, and Brighton is one of them. They are really good at developing young talents, and I think their style of play will fit me a lot."
He made 18 appearances for AIK this season across the league and cup, scoring five goals and providing four assists. He joined AIK last summer from the Ikon Allah Football Academy in Nigeria, where he was first identified.
The move carries sentimental weight. Yohanna described his departure from Stockholm as genuinely difficult.
"I had a lot of good people around me who took care of me. Everyone loved me, so it was difficult to leave."
He met with Hürzeler before agreeing to join.
"He urged me to keep fighting and working hard. I will give everything I have and I really want to win trophies with the team."
AIK confirmed publicly on their stock exchange listing last week that formal negotiations had opened with a club, with the initial bid indicating a net profit effect above £20m. Brighton ultimately prevailed over Newcastle, Chelsea and Tottenham by offering the most convincing footballing project for a player at this stage of his development.