The deadline for Wieters to decide on the option was Monday.
Of his salary for 2018, $2.5 million will be deferred, with $1.25 million due Jan. 15, 2019, and the other half due Jan. 15, 2020.
The 31-year-old Wieters, a switch-hitter, was a four-time All-Star for the Baltimore Orioles before joining Washington as a free agent in February, after spring training camp already had opened.
His contract was worth $10.5 million in 2017, when he batted .225 with a .288 on-base percentage, 10 homers and 52 RBIs for the NL East champion Nationals. He hit .143 during Washington’s loss to the Chicago Cubs in an NL Division Series.