The 26-year-old Salem, Mass. native was acquired by the Blues on Feb. 27, 2017 in a trade with the Washington Capitals.
Last season, Sanford dressed in 52 regular-season games, posting 16 points (10 goals, six assists) and 25 penalty minutes.
In three seasons with St. Louis, Sanford has recorded 71 points (36 goals, 35 assists) in 183 regular-season games.
He also has appeared in 25 playoff games and scored a goal in Game 7 of the Stanley Cup Final to help the Blues capture the first Stanley Cup championship in franchise history in 2019.
The Blues have recently announced new contracts for restricted free agents Jordan Kyrou, Dakota Joshua, Ivan Barbashev and Pavel Buchnevich. Another restricted free agent - Robert Thomas - still remains to be signed.