The 41-year-old former Wales international joins the Sky Bet League Two champions from Port Vale, who are now searching for a replacement.
Page replaces Chris Wilder, who left the Cobblers after leading them to promotion to take up the manager's job at Sheffield United.
Northampton chairman Kelvin Thomas told the club's official website: "We are pleased to have recruited an outstanding young manager like Rob.
"He is very highly regarded and I am looking forward to working with him to take Northampton forward as we prepare for an exciting year in League One.
"We were looking for a manager to come in and continue the good work that has been going on and Rob will be able to apply his League One experience to build on the foundations that have been laid here with this group of players."
Page joined Port Vale as manager in October 2014, avoiding relegation in his first campaign before leading the Valiants to a 12th-placed finish in the 2015-16 season.
Vale chairman Norman Smurthwaite told his club's website: "We are disappointed at Rob's decision to leave but we would like to thank him for his hard work over the last 20 months and wish him the very best for the future.
"The search for a new manager is already under way and we, as a club, will do our utmost to ensure the best candidate for the job is appointed."
Source : PA
Source: PA