The 20-year old will join the League Two club until January, his third loan spell since signing with Boro in 2005 - he has also spent time with Bournemouth and Aberdeen.
Cobblers manager Ian Sampson was confident Walker would prove to be a handy player for the club.
"I think he will prove a great acquisition," Sampson told the club's official website.
"Technically he has a lot of ability and he is really looking forward to showing what he can do.
"I didn't think he would be available but the chairman has done very well for us and Middlesbrough have also helped us out as well.
"He is a player I have known about for a couple of years and with (Northampton assistant coach) Malcolm Crosby coming in, he has just backed up my view of Josh."