Slovakia international Rodak, 20, currently on duty for his country, ended last season on loan at Accrington.
“He will come in and compete with the keepers we have here for a place in the side,” Millers boss Paul Warne told the Sky Bet League One club’s website.
“He has played for Slovakia, so he has a good pedigree and he also did really well in a loan spell with Accrington last season.
“He got rave reviews for his performances last season, and after watching lots of games we really like him.”
Rodak progressed through Fulham’s academy and made his only first-team appearance in the club’s Carabao Cup win at Wycombe earlier this month. He featured on 21 occasions for Accrington last season.
Rotherham later announced the signing of Manny Onariase from Brentford.
The defender, who had a loan spell at Cheltenham last season, has signed a two-year deal with the Millers after joining for an undisclosed fee.
One player leaving Rotherham is Aimen Belaid.
The defender, who made 21 appearances for the Millers, has left the club by mutual consent.
Rotherham later announced the signing of Richie Towell from Brighton on a season-long loan.
Rotherham also let Dominic Ball leave the club, the defender joining Aberdeen until June 9.