Siddiqi underwent a trial period at Sixfields last term and has now penned terms with the League Two club having overcome a hip injury.
The 27-year-old, who started his youth career at Arsenal, has won four caps for his country.
Cobblers boss Aidy Boothroyd said: "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.
"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."