Krul will be contracted with the Magpies until 30th June 2017, after agreeing a five-year extension from the end of the season.
Both players have played key roles in United's fantastic form during the 2011/12 campaign, which sees the Club currently sat sixth in the Premier League table.
Manager Alan Pardew told nufc.co.uk: "These are two fantastic deals for the Club and we are all absolutely delighted.
"To secure new contracts for Fabricio and Tim, two pivotal players at the Club, is terrific news for all concerned, not just ahead of the derby but for the long-term future of the Club.
"It underlines the importance that we as a Club place on building and retaining top class talent, and it demonstrates the intent of Newcastle United to be major players in the Premier League.
"I would also like to place on record my thanks to Mike and Derek for their hard work and continued financial backing in ensuring these deals were completed.
"We now turn our attentions to tomorrow's big game, boosted by this news - which I hope gives our incredible fans a huge lift too - and with the confidence to really make this a season to remember."