The 30-year-old - capped four times by his country - has signed a two-year deal at the New York Stadium, after leaving Wigan Athletic.
He joins Carlton Morris to become Warne’s second close season signing, and will officially join the Millers on July 1.
The former Swansea and Bournemouth central midfielder - who played with boss Warne on loan at Yeovil Town - helped the Cherries to promotion out of League One, before the south coast club moved up to the Premier League.
He has spent the last three seasons at Wigan, but Rotherham have stepped in to offer him first-team football.
MacDonald started for Wigan in their 3-1 defeat last season in the EFL Cup at Rotherham - one of just two appearances for the Lancashire club in 2018-19.