The 25-year-old Australian made 12 appearances for Posh last season, after joining them from Crewe, but was told last month to find a new club.
"Over the last three seasons, Rochdale have almost made it into the play-offs.
"That's what has driven me to this club. Hopefully I can be part of achieving the next step," Inman told the Rochdale website.
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Rochdale have signed midfielder Brad Inman on a season-long loan from League One rivals Peterborough United.