The midfielder has joined League One rivals Doncaster Rovers on a free transfer, signing a three-year deal at the Keepmoat Stadium.
It spells the end of Close's time at his boyhood club, having graduated through the youth ranks.
In total, he made 190 appearances for Pompey, scoring 19 goals.
During his time at Fratton Park, Close helped the Blues capture the Checkatrade Trophy at Wembley in 2019 and clinch successive play-off finishes in the 2018-19 and 2019-20 campaigns.
The Southsea lad was offered a new deal to extend his stay on the south coast.
After Pompey turned down the chance of activating the option of an additional 12 months, they tabled him a two-year contract – but on reduced terms.
Close instead decided to explore his possibilities as a free agent this summer.
Close links up with former Blues team-mate Noel Hunt – who is assistant manager to Richie Wellens – at Donny.
He will return to PO4 with Rovers during the 2021-22 season.