Sunderland were said to be interested in Ladapo, 29, after he handed in a transfer request during the January window.
The striker found the net 11 times in 21 League One appearances this season as the Millers were promoted under Paul Warne – including a brace in October’s 5-1 win over the Black Cats.
Rotherham had 12-month option in Ladapo’s contract which would have extended his deal until the summer of 2023.
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However, the option has not been taken up with Ladapo set to leave the club on June 30 after Rotherham’s retained list was revealed.
The Millers have also released defenders Jake Cooper, Joe Mattock and Angus McDonald and forwards Jacob Gratton and Mickel Miller.
Hull City are now said to be interested in signing Ladapo following his release by Rotherham United.
City finished 19th in the Championship table last year