The player, Martin Sherif, is a 19-year-old striker who has played for Everton's under-18 and under-21 teams.
Rotherham beat other Championship clubs to sign Sherif on a season-long loan.
Sherif is a highly-rated player at Everton, where he scored the most goals for the under-21 team last season.
Rotherham's next match is against Wigan Athletic in the League One on Saturday.