Soto, 22, was originally in Tuesday's lineup against the St. Louis Cardinals, but the Nationals tweeted the IL move about an hour before the game. Soto will have to sit out at least the next seven games; the Nationals have three off-days during his IL stint.
"He was dealing with it over the last few days. A tight tricep," Nationals manager Davey Martinez said. "[An] MRI came back showing a slight strain in his left shoulder.
"We thought we're not going to take any chances with him."
Martinez added he was not sure how Soto injured himself. And the topic of his injury was clearly a talker in the clubhouse among his teammates.
"Hopefully," shortstop Trea Turner said, "we can play well and kind of survive without him."
Outfielder Yadiel Hernandez was recalled from the team's alternate site to replace Soto on the roster. Andrew Stevenson took Soto's spot in right field Tuesday and went 1-for-3 with a run scored. The Nationals won 3-2.
Soto is hitting .300 with two home runs and eight RBIs in 14 games this season.
The Associated Press contributed to this report.