The 21-year-old had been a long-time target of Kenny Jackett following a successful loan spell last season.
The Blues boss made Walkes one of his top targets at the end of the campaign, yet had to wait for Spurs to allow his exit.
Now Pompey have secured the versatile performer on a two-year deal, with the club possessing an option for an additional 12-months.
Walkes' arrival has long been anticipated, nonetheless represents Jackett's sixth signing of the summer.
After arriving in January in a loan deal with Spurs, he established himself as a regular in Pompey's side.
Operating at right-back and then as a holding midfielder, the former Atlanta United player impressed the Blues' boss during 12 successive starts.
However, Walkes' season was cut short after sustaining a hamstring injury during the closing stages of the 3-3 draw at Rochdale in April.
He was unable to feature for the remaining five matches of the campaign, yet stayedÂ at Fratton Park for rehabilitation.
Ahead of his expected move, the Spurs graduate was present for the opening days of Pompey pre-season training, before returning to White Hart Lane.