The 29-year-old has signed a contract running until 2030, returning to Borussia-Park to fill the gap left by Nico Elvedi, who joined Leeds United earlier in the window.
Borussia Monchengladbach sold Itakura to Ajax for around €10.5m twelve months ago, but will pay considerably less to bring him back, with the fee understood to be worth up to €4m including add-ons, plus a sell-on clause for the Dutch club.
Sporting director Rouven Schröder explained the thinking behind the move.
"There was already the idea in winter of bringing Ko Itakura back to Borussia," Schröder said. "What sounded like a bold plan back then has now become reality.
"We are very pleased that we have been able to sign a player who matches exactly the profile we were looking for in terms of quality and experience after Nico Elvedi's departure.
"The fact that he is also a familiar face who will barely need any time to settle in, and who really wanted to come back to us, rounds the whole thing off perfectly."
Itakura first joined Gladbach from Manchester City in 2022 and quickly established himself as a first-team regular, making 80 competitive appearances and scoring seven goals before his move to Ajax last summer. The Japan international, capped 42 times by his country, is expected to slot straight back into the heart of Gladbach's defence and could make his second competitive debut for the club in the DFB Cup first round this weekend.
It represents smart business overall for Gladbach, who have solved their most pressing defensive need at a fraction of the fee they received for Itakura only a year ago.
Itakura's path back to Germany began at Manchester City, who signed him from Japanese club Kawasaki Frontale in January 2019 before loaning him out for spells at Groningen and Schalke, where he scored four goals in 31 second-division appearances during the 2021/22 promotion season.
He never played a senior match for City, and his move to Gladbach in the summer of 2022 proved the making of his European career, with the defender becoming a consistent performer in the Bundesliga before Ajax's move for him last year.
Schröder's admission that the club had been eyeing a reunion with Itakura as far back as the winter transfer window suggests this was less an opportunistic deal and more the culmination of a plan that had been quietly in motion for months.