The 27-year-old, out of contract in the summer, was linked with several clubs including Portsmouth and Charlton Athletic - with the clock ticking down regarding his future.
Sadlier, who joined the Millers from Doncaster Rovers in the summer of 2020, has made 21 appearances in all competitions for the club this term, scoring six times - including a treble in the EFL Trophy game with Manchester City under-21s.
His departure follows on from the exit of Curtis Tilt to Wigan Athletic.
Millers manager Paul Warne said: "He's a really good footballer, a really technical player. Just because it doesn't work here for him doesn't mean he's not going to have an amazing career somewhere else. He did really well at Doncaster Rovers and he's done really well in certain games for us.
"It's just about the game-time. He's at an age where he needs to play. He's been in my office probably six times this season telling me how much he wants to play. His attitude has been spot on. He's always been patient.
"He was always aware of Bolton's interest in him. It was something he wanted to do, it wasn't something I wanted to do. The longer the season went on and the less game-time he got, the more frustrated he became. I don't want a hostage here.
"He's probably just outgrown his time here. Had I kept him beyond the window and then still played him one game in six, would I have had the same Kieran? I don't think I would have. I'm not saying he would have downed tools, I just don't if he'd have had the full level of motivation we need to get promoted. Sometimes you can see it in a player's eyes: 'Please, I just want to move on and play in a different system.
"He didn't want to play as a wing-back for me. He made that clear. I didn't sign him as a wing-back but since I signed him we've changed system."
On the incoming front, Rotherham are hoping to complete the signing of former Bohemians striker Georgie Kelly, who was undergoing a medical at the club today (Friday).
Alongside Sadlier, a number of other Millers players are out of contract in the summer including Chiedozie Ogbene, Michael Smith, Jamie Lindsay, Michael Ihiekwe, Viktor Johansson, Joe Mattock, Josh Kayode and Mikel Miller.
Freddie Ladapo is available for transfer after tabling a transfer request early this month which was accepted. So far, there have been no offers for the 28-year-old.