The team said on its Web site the lucrative deal allows both sides to avoid salary arbitration for the next three seasons.
The burly first baseman batted .251 last season for the World Series champions with a major-league leading 48 home runs and 146 RBI.
The figures helped him finish second to St. Louis first baseman Albert Pujols for National League MVP honors.
"I'm happy to have this done and to know that I'll be in Philadelphia for at least another three years," Howard said in a statement. "Both sides are happy and now I'm just focused on getting the season started."
"Ryan is clearly one of the top power hitters and run producers in the major leagues and is a very important part of our championship club," said General Manager Ruben Amaro Jr.
Last spring, Howard won $10 million, the largest sum ever awarded to an arbitration-eligible player.