Brigham and Women's hospital in Boston.
Kraft, 61, went in for a routine checkup when an angiogram revealed a heart condition that required immediate surgery.
He is now is now resting comfortably and his doctors expect him to make a complete recovery.
Kraft, who purchased the Patriots in 1994, is expected to make a full recovery, the team said in a statement.
The Patriots open defense of their surprise Super Bowl title Sept. 9 against the Pittsburgh Steelers.