The 41-year-old former Wales international will leave Port Vale, who finished 12th in League One this season, with one year left on his contract.
Northampton will pay compensation to the Valiants but that will not be too much of a problem after Sheffield United's deal to take Chris Wilder from Sixfields.
Page had also been linked with the Cardiff City job recently and Port Vale midfielder Anthony Grant had not been surprised, telling BBC Radio Stoke: "The players haven't got a bad word to say about him. Even the ones who aren't playing. That says everything about him."