The defender made 68 appearances for the Dons between 2009 and 2015.
He moved to Blackpool in 2015 before joining Rotherham in 2018. The Millers have just been relegated from the Championship but were hoping to keep Robertson at the club.
Celtic captain Scott Brown and Motherwell skipper Declan Gallagher have already signed pre-contract agreements to join the Dons this summer.
The Reds have also been linked with moves for St Mirren’s attacking midfielder Jamie McGrath and Livingston forward Jay Emmanuel Thomas.
Glass has said the club will announce which of Aberdeen’s out of contract players will be offered new deals next week.
Aberdeen hold talks with defender Clark Robertson over Pittodrie return