The two teams had settled on a fee but the 23-year-old had still to discuss personal terms and undergo a medical, Everton announced on their website (www.evertonfc.com).
Fer, who began his career at Feyenoord, has made two full appearances for his country.
Everton, fifth in the table, entertain eighth-placed West Bromwich Albion in the league on Wednesday.
Twente are second in the Dutch top flight, one point behind PSV Eindhoven.
- Sports & Recreation