The left-back has arrived at Fratton Park for a month and goes straight into the squad for Tuesday's trip to Stevenage.
The 25-year-old made 39 npower Championship appearances during a spell at Pompey during the 2010-11 season and admitted the move was a "no-brainer" for him.
"It's an exciting move," he told the club's official website.
"I wasn't getting a chance at Watford and I needed games, so I jumped at the chance to come back.
"I know what it's like to play at Fratton Park and know how great the fans are - hopefully they can push us on."