The giant former Manchester City stopper has signed a three-year contract with Forest that promises to keep him at the club until the summer of 2021.
Pantilimon had played just nine times for Watford during his time at Vicarage after joining the club from Sunderland in January 2016. He spent the first half of last season on loan in Spain with Deportivo La Coruna prior to arriving in Nottingham.
The 31-year-old, who has 31 international caps for Romania, made 13 appearances in the Championship and kept four clean sheets for a Forest side that finished the season in 17th.
Forest have already had a busy summer so far after signing former hero Michael Dawson, Portuguese duo Joao Carvalho and Tobias Figueiredo, ex-QPR and Liverpool defender Jack Robinson and Algerian international Hillal Soudani.
Aitor Karanka's team will begin the new Championship season on 4th August when they travel to face Bristol City at Ashton Gate.