Span will make $9 million in 2015.
The Nationals declined the options of outfielder Adam LaRoche and pitcher Rafael Soriano, according to Comcast Sports Net's Mark Zuckerman.
"Denard was arguably our most consistent offensive player in 2014, keying so much of our offensive production, and his defense in center field is Gold Glove-caliber on a nightly basis," said general manager Mike Rizzo.
Span, who just completed his seventh MLB season, batted .302 with five home runs and 37 RBI. He also led the National League with 181 hits and stole 31 bases, the highest single-season total of the outfielder's career.
The 31-year-old will be a free agent at the end of the 2015 season. Washington acquired the longtime Minnesota Twin in a November 2012 trade.
The Nationals went 96-66 on the regular season and won the NL East before losing in the NLDS to eventual World Series champion San Francisco.
- Will Green