Twenty-two-year-old Middleton has impressed during his initial five appearances for the Pilgrims and will now be with United up to and including the Leamington home fixture on February 17th.
Middleton (pictured) signed for Harrogate in June following a season in the National League with North Ferriby United.
Middleton was also a member of Ferriby’s 2015/16 promotion-winning squad, who he joined after two seasons and seventy-four appearances at Scarborough Athletic.
Middleton previously spent time in the youth systems of both Doncaster Rovers and Hull City.
Full-back Paul Walker, meanwhile, has re-joined former club Spalding United on dual terms, with the Pilgrims retaining the player's National League registration.
Former Grimsby Town trainee Walker linked up with Boston in December.
The right-back had been with Spalding in the Evo-Stik League South but moved back up to join the Pilgrims following previous experience of National League North following a loan spell with Harrogate.
The 22-year-old has made four appearances for the Pilgrims to date.