The 21-year-old has committed to a three-year contract with the Rotterdam club.
Feyenoord technical director Martin van Geel said: "Warner has made a big impression with his performance in Dordrecht.
"With us, he gets the chance to show his skills on a higher stage and he goes into battle with the other keepers in the squad."
Hahn added: "I see this as a very good step in my career.
"I was born and raised in Rotterdam and I currently live five minutes from De Kuip. When Feyenoord plays at home I always see the stadium lights lit.
"I will fight for my place, but first, I focus the rest of this season completely with FC Dordrecht."
Hahn, who had been linked with a move to Newcastle United, has been capped seven times by the Netherlands at Under-21 international level.