The Philadelphia Phillies officially year, $50,000,058 million contract Monday.
The deal has a vesting option for the fifth year.
"We are really pleased to bring Jonathan to Philadelphia," said Phillies general manager Ruben Amaro Jr. "He is among the elite closers in the game and someone who clearly has a passion to win."
Papelbon, who will turn 31 years old November 23, has been Boston's closer for the past six seasons. He helped the Red Sox to a World Series title in 2007 and was selected as an All-Star four times (2006-09).
The right-hander posted a 2.94 earned run average in 63 appearances last season, and converted 31-of-34 save chances.
He has spent his entire career with the Red Sox since they selected him in the fourth round of the 2003 draft. Papelbon holds a 2.33 ERA in 396 career games, and has converted 219-of-248 save opportunities. He has also struck out 509 and walked 115 in 429 1/3 innings.
Papelbon will replace Ryan Madson, who was drafted by the Phillies in 1998 and has spent most of his career as a set-up man. However, he was their closer last season and posted a 2.37 earned run average in 62 appearances, while converting 32-of-34 save chances.
Papelbon will wear No. 58.