The Saints allowed North to feature in Wales' 30-26 defeat at the hands of Australia on November 30, which came after the end of the IRB autumn Test period.
Premiership clubs have an agreement that only England players can be released for matches or training camps outside the IRB window.
But wing North, 21, ensured he would be available for all Wales fixtures when he negotiated his move from the Scarlets to Franklin's Gardens in the summer.
Saints appeared before a three-man panel of club chief executives on Wednesday, and were handed the fine after admitting a breach of Premiership Rugby Board policy. Northampton will not appeal the decision.
A Premiership Rugby statement read: "A meeting took place this morning of the Designated Panel appointed by the Premiership Rugby Board under its Code of Conduct to consider the charge against Northampton Saints that the club had breached the Board policy with regard to international player release.
"Northampton Saints admitted the breach with respect to the Board policy through the release of George North beyond the requirements set out in IRB Regulation 9.
"The Designated Panel, comprising three Board members appointed under the terms of the Code, decided that Northampton Saints will be fined £60,000.
"Northampton Saints have decided not to appeal against the sanction.
"There will be no further comment from Premiership Rugby or Northampton Saints."
The punishment is more severe than had been expected and could be seen as a deterrent to clubs taking similar action in the future. Gloucester, for example, recently announced Wales hooker Richard Hibbard will join them next season.
When North was picked in the Wales side to face Australia, Northampton had acknowledged their agreement with the winger was ''outside Premiership Rugby Board policy'', but insisted it had ''not contravened any rules, laws or regulations''.