This is Gore's second stint with the Millers, having previously played for the club in a similar arrangement last season.
During his time with Manchester United, Gore has made a few appearances, including his senior debut in a Carabao Cup match against Crystal Palace in September 2023.
He has also played in the Premier League for Manchester United and gained experience in the English Football League (EFL) with a loan move to Port Vale.
Gore's loan to Rotherham United comes after a successful rehabilitation with Manchester United's Under-21 team, which allowed him to recover from an injury that cut short his previous loan spell.