Brandt, 30, arrives on a free transfer after his Borussia Dortmund contract expired in June, the 48-time German international putting pen to paper on a three-year contract in Amsterdam.
The move was a surprise to many, with all eyes set on the pending arrival of Marc-Andre ter Stegen from Barcelona.
"Julian has chosen Ajax, and we are absolutely delighted about that," technical director Jordi Cruyff said on the club website.
"We have been in talks for several months, and he also had the opportunity to sign for other clubs. 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."
Brandt joined Dortmund from Bayer Leverkusen in 2019 and played 307 games for BVB across all competitions, scoring 57 goals and providing 65 assists. Last season, Brandt appeared in 41 games for Dortmund, recording 11 goals and 4 assists.
Brandt has become the fifth summer arrival at Ajax, where Jordi Cruyff is working hard on building a new squad. Daley Blind, Caio Henrique, Marcos Leonardo, and Tolu Arokodare joined Ajax earlier in the summer.
The Amsterdam outfit are coming off the back of their joint-worst league finish since 1963, finishing fifth with 63 points and only reaching the UEFA Conference League qualifiers via the play-offs.
Ajax hired former Girona manager Michel in June and have reached the third round of the UEFA Conference League qualifying phase after beating Serbian outfit Vojvodina Novi Sad 8-2 on aggregate.
