According to CNN, the actor is suing Screen Gems, CRT Holdings, Sony Pictures Television and Sony Pictures Entertainment in Los Angeles Superior Court.
"For nearly 40 years, defendants have swindled Mr. Cassidy out of his rightful share of the profits from 'The Partridge Family,' and when Mr. Cassidy has inquired as to the matter, have lied to him so as to continue to conceal their deception," the lawsuit says.
Additionally, the lawsuit alleges that the defendants aren't staying true to Cassidy's contract and are continuing to scam him.
CNN reports the lawsuit claims fraud and breach of contract and the defendants owe Cassidy millions.
"It's just a matter of being fair, and doing the right thing. I have no other alternative if Sony is not going to be reasonable," Cassidy told CNN. "I'm asking for fairness, I'm asking to be treated with respect as opposed to, 'we don't owe you anything."
Cassidy said his former manager told him he's received $5,000 of merchandise profits. However, Cassidy's 1971 contract with Screen Gems is said to give him 15 percent of net merchandising revenues for the use of his image, voice or likeness. He's also entitled to no more than half of that for items that include other cast members.
'The Partridge Family' is just one of many former sitcom stars who have sued their parent companies for merchandise profits. James Best of 'Dukes of Hazzard' fame recently filed suit against Warner Bros., claiming to be owed $25 million for royalties. 'Happy Days' actors Anson Williams, Don Most, Marion Ross, Erin Moran and the estate of the late Tom Bosley filed suit against CBS in April. The 'Happy Days' cast is suing for $10 million in merchandising revenues.