The midfielder will join Rotherham from Brighton on a one-year loan.
The 22-year-old Englishman played last season 50 times for his club. He also played for Oxford United and Milton Keynes Dons before.
Brighton's manager Chris Hughton said:
"An opportunity has arisen for Jake to gain regular Championship football again, which benefits all three parties."
Forster-Caskey scored nine goals in 82 apperances since he joined the club in 2009.