The move, agreed at the start of August but delayed while Tottenham reconsidered releasing him, is a straight loan with no purchase option attached.
Moore's strong start to the season — he started both of Spurs' opening Premier League fixtures and scored against Charlton Athletic in the Carabao Cup — had prompted manager Roberto De Zerbi to weigh keeping him in north London before ultimately allowing the move to proceed.
Cologne sporting director Thomas Kessler was enthusiastic about the signing.
"Despite his young age, Mikey already displays remarkable maturity," he said. "He possesses excellent technical ability, is creative and bold in one-on-one situations, and consistently poses a threat in attack thanks to his dynamism and carefree style.
"His profile will expand our options in the final third and give us added versatility. We are convinced that he will take the next steps in his development with us."
Moore, who joined Tottenham's academy as a seven-year-old growing up in south London, made his Premier League debut against Manchester City at Tottenham Hotspur Stadium in May 2024, becoming the club's youngest-ever player in the competition at 16 years and 277 days old.
He featured in the side that won the Under-17 and Under-18 Premier League Cup double in 2022-23, and scored his first senior goal during Tottenham's run to Europa League glory in 2024-25 — a display against AZ Alkmaar that prompted team-mate James Maddison to remark he thought Spurs "had Neymar on the wing."
Last season was spent on loan at Rangers, where Moore contributed seven goals and four assists in 47 appearances and was named the Scottish Premiership's Young Player of the Year. He joins fellow English loanees Reigan Heskey and Jahmai Simpson-Pusey at the Rhineland club, and could make his Bundesliga debut when Cologne travel to face VfB Stuttgart this weekend.
He also becomes only the latest in a long line of English players to move to Cologne, following in the footsteps of Tony Woodcock, who became the first official English signing in Bundesliga history when he joined the club in 1979.
Tottenham's business on deadline day extended beyond Moore. Brazilian full-back Souza has also left the club on loan, joining Portuguese side Porto for the rest of the campaign, while defender Kevin Danso is separately expected to depart after asking to leave, and several Tottenham under-21 players — including Alfie Dorrington, James Rowswell, Tyrese Hall and Damola Ajayi — are also poised to head out on loan to EFL clubs before the domestic deadline.