The 21-year-old travelled to Limburg on Monday for a medical even before the two clubs had finalised the agreement, eventually completing the move as Tottenham's business wound down on deadline day.
Takai will spend the rest of the season in the Jupiler Pro League after failing to force his way into contention at Tottenham following a big-money move from Kawasaki Frontale in the summer of 2025.
Sint-Truidense host Royale Union Saint-Gilloise later on Wednesday, though Takai will not be eligible to feature in that fixture, according to Het Belang van Limburg. Whether he is available for Saturday's trip to RAAL La Louvière remains unclear, with the registration process not yet fully resolved at the time of the loan being announced.
Head coach Frédéric De Meyer said he was excited by the calibre of player his club had brought in.
"Kota does indeed remind me a bit of Shogo Taniguchi, but much younger," De Meyer said. "I think that in the long run he could even become Shogo's successor in the Japanese national team."
Takai's route to Belgium has been far from straightforward. Tottenham paid around €6million to sign him from Kawasaki Frontale, but an ankle injury kept him sidelined for five months before he ever made his first-team debut, and he was subsequently sent on loan to Borussia Monchengladbach in January, making just eight Bundesliga appearances during that spell.
An Asian Under-23 Cup winner and 2024 Olympian with Japan, he has earned four senior caps for his country and will now look to rebuild momentum in Belgium after a difficult opening period following his move to Europe.
De Meyer's comparison to Shogo Taniguchi, a mainstay of Japan's national defence for the best part of a decade, reflects the level of long-term potential Sint-Truidense believe they have secured on loan, even if short-term involvement will need to wait for administrative clearance. For Tottenham, the move continues a pattern of sending young academy or recently signed players out on loan to gain experience elsewhere, having already committed significant resources to a defence that has left De Zerbi with little room for a player still adapting to European football.