The defender arrived for a reported fee of 15 million euros, breaking the club's transfer record. Gosens' signing represents a coup for Union, who were promoted to the top division for the first time in 2019 and qualified for the Champions League this season after a fourth-placed finish last year.
"The last few years at Union have been impressive and people abroad have noticed that," Gosens said in a statement released by the club. Gosens said he felt "appreciation and trust" when talking to club officials and explained "the decision wasn't hard for me to make."
Union sporting director Oliver Ruhnert said Gosens stood for "commitment, mentality and team spirit." "On and off the pitch, he will now play a leading role for us." German media reported on Tuesday Gosens moved to Union in search of more playing time ahead of Euro 2024.