The deal is worth an average of $6.5 million a year, a source told ESPN.com's Pierre LeBrun.
The 23-year-old Pietrangelo, the No. 4 pick in the 2008 draft, has been part of St. Louis' top defensive pairing the last three seasons. He is also the Blues' top scoring threat on the back line.
Pietrangelo had five goals and 19 assists in the lockout-shortened 2012-13 season. He had 12 goals, 51 points and a plus-16 rating in the 2011-12 campaign.
The Associated Press contributed to this report.