However, negotiations hit a snag earlier in the week, only to resume this morning, upon which the two sides came to terms.
"We're excited to have Alex in the fold for the next seven years as he will continue be a key component of our franchise," said Blues GM Doug Armstrong. "He is an elite defenseman in this league and will be for years to come."
Pietrangelo, 23, dressed in 47 games last season, leading all team defensemen with 24 points (five goals and 19 assists) while ranking 13th in ice time per game (25:06) among all NHL defensemen.
In 2011-12, Pietrangelo joined Al MacInnis as the only defensemen in club history to record six game-winning goals and six powerplay goals in the same season.
Over his young career, Pietrangelo has appeared five NHL seasons, accumulating 29 goals and 92 assists for 121 points, along with 73 penalty minutes in 224 career regular season games.
The King City, Ontario native was originally drafted fourth-overall by the Blues in the 2008 NHL Entry Draft.