The Bulgaria Under-21 international has put pen to paper on a four-year deal at the City Ground.
He said on Twitter: “I am happy to announce I’ve signed a new long-term deal with the club! It means so much to me and my family! Hard work carries on! #nffc.”
Evtimov made his debut for Forest at Leeds during the 2013/14 season and has previously spent time on loan with Nuneaton Town and Mansfield.
He is expected to provide back-up to Dorus de Vries and Ben Hamer, who joined the club on loan from Leicester at the weekend.