Last season Brandt scored seven goals and added four assists in 29 league appearances, 17 of them starts, continuing to feature prominently for Borussia Dortmund despite the uncertainty hanging over his future.
Ajax confirmed the deal through their official channels, with technical director Jordi Cruyff explaining the thinking behind a signing that had been in the works for months.
"Julian has chosen Ajax, and we are absolutely delighted about that. We have been in talks for several months, and he also had the opportunity to sign for other clubs," Cruyff said.
"With Julian, we are bringing in a top international player with a wealth of experience and a fantastic mentality.
"I am convinced he will add real quality to our squad."
Leeds United had also tabled a three-year contract offer and were considered ahead of Ajax for a period, but Brandt ultimately opted for Amsterdam, penning a deal that runs until the summer of 2029. The attacking midfielder becomes the fifth summer arrival at the club, following Daley Blind, Caio Henrique, Marcos Leonardo and Tolu Arokodare through the door as Cruyff continues to overhaul the squad following Ajax's joint-worst league finish since 1963 last season.
Brandt, capped 48 times by Germany and a Champions League finalist with Dortmund in 2024, said his relationship with Cruyff was decisive in the decision.
"I have been in contact with Jordi since May. At that time, he told me that Ajax was interested. I was obviously very happy, because Ajax is a traditional, great, and big club," he said, adding that conversations with the club's coaching staff about their playing philosophy also swayed him.
"He wants to play the kind of football that belongs at Ajax. Making combinations, scoring lots of goals, and ultimately being successful. That suits me. I have the feeling that the plan fits me perfectly."
Across 307 appearances for Dortmund in all competitions, Brandt scored 57 goals and provided 65 assists.