Wilder opted to leave Oxford, currently in a play-off place in League Two, after five years in charge in favour of signing a three-and-a-half-year deal at Sixfields, the two clubs having agreed a compensation package over the weekend.
Northampton are bottom of the Football League and six points adrift of safety, but Wilder - who guided Oxford into League Two via the play-offs in 2010 - is being backed to turn around the club's fortunes.
"I am delighted to be able to appoint Chris," said Northampton chairman David Cardoza.
"We were looking for a manager who has the experience required to guide us out of our current position and one who is ambitious and driven enough to take the club forward in the medium term and Chris is the perfect choice.
"The fact that he served Oxford and Halifax (who he managed between 2002 and 2008) for so long shows what a good job he did at both clubs.
"Chris will be able to strengthen the squad before the transfer window closes and that is now the focus of our work."
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