The contract, which has an average annual value of $1.5 million, allows the 24-year-old forward to avoid an arbitration hearing.
Sanford had 20 points (eight goals, 12 assists) in 60 regular-season games, and four points (one goal, three assists) in eight games during the Stanley Cup Playoffs to help the Blues win their first championship.
Selected by the Washington Capitals in the second round (No. 61) of the 2013 NHL Draft, Sanford has 28 points (12 goals, 16 assists) in 99 games with the Capitals and Blues.