Owen and Karl Oyston are seeking Â£150,000 damages from the Back Henry Street website.
Papers were served on Thursday over six alleged defamatory comments which were made in 2014.
A club spokesman confirmed the legal action but said it would not be commenting further.
A notice published on the fans' website stated: "To clear up any rumours, on Thursday 5 February 2015, Back Henry Street was served papers from the court, relating to the Oystons' and Blackpool Football Club's intent to seek damages for six allegedly defamatory comments made on the site in 2014, limited collectively at Â£150,000.
"For obvious reasons, we can not go into any more detail at this time. Thank you for your continued support."
The forum has appealed to fans of the Championship side to help in its legal fight.
One of its moderators posted a message saying: "A football club exists and prospers due to the support of its fan-base and Back Henry Street will forever champion the freedom of thought and expression in discussion of Blackpool Football Club.
"It is vital we ensure that we, the real Blackpool fans, will always have this forum to discuss our club."
Last week dozens of Blackpool fans attended a court hearing in Manchester for a separate legal action brought by the Oystons against a fan.
David Ragozzino, 32, was given 21 days to respond to a claim for damages from the club.
On 30 January another fan, Stephen Sharpe, apologised to the Oystons over his online comments, withdrawing his posts after accepting they were "false, defamatory and, in some cases, threatening".
The club also has legal action pending against the former chair of the Blackpool Supporters' Trust, Tim Fielding.
The owners of Blackpool Football Club are suing a web forum for libel in the latest of a series of actions against fans making derogatory comments online.