Maddison is regarded as one of the best players at the third tier of English football, and the 24-year-old netted eight goals in 41 appearances in League One this season.
However, owner Darragh MacAnthony has revealed that he promised to give the playmaker a chance to sign for a bigger club this summer on the basis that a three-year contract was penned before the current campaign.
MacAnthony told the club's official website: "When I sat down with Marcus last August and agreed a new contract with him, I gave him my verbal assurances that were we to fall short and not gain promotion, I would allow him to leave the club subject to an offer acceptable to the football club and place him on the transfer list.
"Marcus has been in his four years with the football club our stand out attacking midfielder and one of the best players offensively outside of the Premier League and the Championship.
"He has had an incredible career with the Posh and it is only right that I stick to my word and give him the best chance possible to further his ambitions of playing further up the football pyramid so with sadness today, I can confirm he is available for transfer."
In total, Maddison has scored 42 goals in 177 appearances for Posh.