Fabbri, who was a restricted free agent, has been plagued by injuries over the last two seasons. He returned in 2018-19 after a pair of knee surgeries and separated his shoulder in December.
The 23-year-old collected six points in 32 regular-season contests for the Blues this past campaign before scoring one goal in 10 playoff games en route to the Stanley Cup championship.
Fabbri racked up 15 points across 20 postseason contests as a rookie in 2016.
He's spent all three of his NHL seasons with St. Louis, which drafted him 21st overall in 2014.