The 26-year-old moves to the Rhineland with Köln also securing a purchase option believed to be worth €11 million, giving the Bundesliga club the chance to make the deal permanent depending on how the loan spell goes.
Dallinga will compete with Ragnar Ache, 28, for the central striker's role at Köln. Bologna paid a hefty €16 million for the one-cap Dutch international in 2024, and the forward will now be hoping to rediscover his best form in Germany after a difficult spell in Italy.
Köln were not the only Bundesliga club interested in Dallinga, with Werder Bremen also credited with interest in the striker. In the end, though, it was their league rivals who won the race for his signature.
Köln sporting director Thomas Kessler welcomed the arrival with clear enthusiasm.
"We're very pleased that Thijs has chosen FC. He's a striker we've watched closely and who brings many of the qualities we were looking for in our attack.
"Thijs has a strong presence in the penalty area, good movement in front of goal, and at the same time works very hard for the team without the ball."
Dallinga arrived at Bologna from Toulouse in 2024 after an impressive spell in Ligue 1, where he scored 32 goals in 37 appearances during his breakout 2021-22 campaign before further productive seasons in France.
His time in Italy never lived up to that early promise, though, with persistent fitness issues and limited game time restricting him to a modest return in Serie A. Köln will be hoping a change of scenery and a fresh role at the sharp end of their attack can help him rediscover the form that first made him such an attractive target across Europe.