Alnwick had been on trial at the Tangerines but once he learned of interest from Posh, the former Sunderland and Tottenham goalkeeper did not hesitate to yes to a move to London Road.
He told the club's YouTube channel: "I was all set to sign for Blackpool but when I heard about Peterborough, it was a no-brainer. I know the goalkeeping coach from Spurs and I've heard great things about the manager and the club.
"When I heard the deal was on I was over the moon, I came straight down to sign.
"I need to get playing now. I was enjoying my time at Charlton before the takeover went through and a few of us were on the wrong end of it. Now I'm ready to kick on again and get this club to where it belongs."After a series of loan moves in his career, Alnwick is looking forward to getting settled at his new club, after signing a three-year deal with Darren Ferguson's side.
He added: "A lot of the moves I went on from Tottenham were no more than three-months long and it's hard to get settled. I can now get my family down here and have a good go at it and focus on my football.
"A promotion in my first year would be fantastic. I know you have to take every game as it comes but that has to be the objective for the season to help the club get back in the Championship."