The 25-year-old was offered a new deal by St Johnston but it looks like the Millers have beaten off interest from both side of the border to secure his signature.
The defender made 104 appearances for Saints in two-and-a-half years – and played a key role in the side that claimed a historic cup double in 2020/21.
A statement on the Millers website said.
“Whilst terms are yet to be formalised between the Millers and the player, the club have a provisional agreement in place, which they hope to be finalised in the early stages of next week.
“The Millers will await the outcome of a full medical, whilst also being required to seek approval from the FA, EFL and obtain international clearance in order to ensure his registration is completed.”