The deal, which was agreed before this summer's World Cup, ties the Argentina international to the Nou Camp until June 30, 2018, with the option of a further year on top of that.
Mascherano, whose new contract has a buy-out clause of 100million euros (£80million), joined Barca from Liverpool in 2010 for a reported £18million and has won eight titles, including two Primera Divisions, the Champions League and the Copa del Rey, during his time in Spain.
The versatile 30-year-old midfielder said: "I'm committed to this club until the final day of the contract. I hope I can live up to expectations of the club.
"I'm really happy to be in this great club and be part of a squad with such great players. Let's hope things go really well for us this season. We're looking forward to this season after some difficult times last campaign."
Another player United can release they looked at and missed out on then...- shea666