The 28-year-old spent four years at Leipzig before joining Chelsea in the summer of 2023, and made the switch to AC Milan last year, scoring eight goals and adding three assists in 35 appearances across all competitions.
His spell in Serie A lasted only a single season before the German club moved to bring him back, with the loan agreement including a buy option worth a reported €28m.
Nkunku had been strongly linked with a host of clubs during the transfer window, including Newcastle United, Aston Villa and Galatasaray, but RB Leipzig's sporting director Marcel Schäfer revealed the move only came together because of the player's own determination to return.
"The transfer wouldn't have happened without Christo's persistence. Two Premier League clubs were prepared to go to the limit — we wouldn't have had a chance," Schäfer told German outlet Bild.
According to the report, the two Premier League sides interested were Newcastle and Manchester United, but Nkunku turned both down, telling the clubs he only wanted to join Leipzig. It underlines the strength of his connection to the Bundesliga side, where he established himself as one of Europe's most productive attacking players before his move to England, scoring 70 goals and winning two German Cups across 172 appearances.
Nkunku's time at Chelsea and Milan since leaving Leipzig has been far less prolific than his first spell in Germany, with persistent injuries during his three years in London limiting his impact before a switch to Italy failed to reignite his career. His return to a club and city he knows well is intended to offer the platform for a fresh start, with Leipzig hopeful his familiarity with the squad and coaching staff will accelerate his return to form.
The move continues a busy summer of incoming and outgoing business for Leipzig, who will be looking to Nkunku's experience and versatility across multiple attacking positions to strengthen a squad aiming to build on recent progress in the Bundesliga.
Milan, for their part, move on from a forward who never quite settled in Serie A despite his pedigree, freeing up space in their attacking ranks while retaining the option of a modest sell-on should Leipzig eventually make the deal permanent.
For Nkunku, the move represents a chance to rediscover the form that once made him one of the most sought-after attacking talents in Europe, in familiar surroundings where he already knows what is expected of him.