A one-month loan deal has been agreed for the 24-year-old who will replace Saido Berahino in the Northampton squad.
Berahino is back with West Bromwich Albion and could soon be joining a League One side on loan.
Manager Aidy Boothroyd, who has confirmed he is not looking to offload Jake Robinson, believes former non-league forward Williams will bring 'power and pace' to his side.
"I'm far happier with the squad than I've ever been," Boothroyd told BBC Look East.