The Pakistan international had a trial with the Cobblers last season, but a hip injury prevented him from joining on a permanent basis.
However, having made a full recovery, the 27-year-old has now been given the chance to "impress" at Sixfields.
"We are giving Kash a chance. We are giving him an opportunity to impress. He has battled back well from injury and is now at the stage where he is training regularly," manager Aidy Boothroyd told the club's official website.
"We've given him a chance to train with the first team and he is very keen and he's very enthusiastic so we will monitor his progress and see how he goes."
London-born Siddiqi started his career as a trainee with Arsenal.