The 22-year-old midfielder, who had a loan spell at Sixfields cut short by injury last season, has penned a deal until the end of the season.
"I am really pleased to bring Josh back to the club," manager Ian Sampson told the club's official website. "We know there was a lot of interest in taking Josh to the end of the season, including from higher division clubs, so we are delighted he has chosen to come back to us.
"He enjoyed his brief spell here with us last season, and he has worked with Malcolm Crosby in the past, so he knows the way we do things.
"We didn't get to see the best of Josh before, his loan spell with us last season was ended because of injury and that was really unfortunate but I know what a good player Josh is and he will be a big asset to us between now and the end of the season."
Walker will be available to make his second Cobblers debut in Tuesday's game against Crewe at Sixfields, a match that will also see Shaun Harrad make his first Town appearance.
Source: PA
Source: PA