The 21-year-old joins Posh on a three-year deal for an undisclosed fee.
Tomlinson came through the Southampton Academy and had spells at Yeovil, Brighton and a loan at Bognor Regis.
He then joined Eastleigh where he scored 12 goals and kept 15 clean sheets in 42 appearances, earning Young Player of the Year and Player of the Year awards at the club.
Manager Darren Ferguson told the club website: “It is a really good signing for us. There were so many teams showing an interest, so delighted to get the deal done now. I was made aware of him last season, we had eyes on him in the first pre-season game and we didn’t need to see him again, it was done, the decision was made.
“He is an excellent athlete, both footed, can play left back or left-wing back or right back or right-wing back, is a good size and is a lovely lad. We see real potential with this one and I am excited about it. He is a modern-day full back, good crosser of the ball with both feet, takes penalties with his right-foot and free-kicks with his left.”
Tomlinson said: “I am really pleased to be here, once I heard about the interest, I wanted to make the move and I am looking forward to it.
“I guess you could say I am a modern day defender in the sense that I enjoy the attacking side of it as well as the defensive side of it. I can’t wait to get started.”
Tomlinson will wear the number 21 shirt.