According to multiple media reports, the forward will not sign an extension with the Bruins and has told the team he intends to explore unrestricted free agency.
The 28-year-old from Welland, Ont., came to the Bruins in a 2010 trade with the Panthers, after spending his first five years of his NHL career in Florida.
Horton's expiring six-year deal was worth a total of $24 million (US), with a final season total of $5.5M and an salary cap hit of $4M.
He scored 56 goals and had 107 points in 169 regular season games with the Bruins, and added seven goals and 12 assists in Boston's 22-game playoff run in 2013.
Horton was also a part of the 2011 Stanley Cup-winning squad, but he was knocked out of the final against the Vancouver Canucks after suffering a concussion from an Aaron Rome hit in Game 3 of that series.
That injury followed him into the next season, where he skated in only 46 games of the 2011-12 season. Another concussion forced him to miss the playoffs in 2012.