The 33-year-old, a free agent after leaving Bramall Lane at the end of last season, has penned a one-year deal with the League Two side.
He told Pompey’s website: “I’m happy that it’s all done. Now I want to crack on and try to help this club move back up through the divisions again.
“This is a massive club and if you achieve something here then you’ll always be remembered.
“There’s a lot of pressure when you’re at a big side and I’m sure it will be no different here, but that’s something I enjoy.
“People will make us favourites to win promotion, so we need to make sure we can live up to those expectations.
“The club are making the right statements off the pitch and now it’s up to us to do a job on it.”