Ryan, who left Belgium's Club Brugge in 2015 for a six-year-deal in Spain, has dropped to third in the pecking order behind fellow glove men Diego Alves and Daume Domenech in La Liga and has been courted by clubs in Spain and across Europe with offers of first team football.
But, after long negotiations between Ryan's agent Mikkel Beck and Valencia chiefs, he was released to join the three-time Belgian champions.
"We think this is a good move for Maty," said Beck.
"Genk are in the semi-final of the cup, they are still in the Europa League and with Maty in the team can hopefully go on to gain a Play-off 1 spot in the league," said Beck.
Ryan is expected to make his debut on Wednesday (AEDT) at home to KV Okstende in the Belgium Cup - a competition Genk have won on four occasions.
Genk lie eighth on the ladder with 34 points from 23 games and Ryan has been earmarked as their No 1 as the previous starting 'keeper, Marco Bizot, is out with injury for at least the next six weeks.
Genk are also in the last 32 of the Europa League, where they face Astra Giurgiu on February 18.
Ryan made just two league starts for Valencia this season.