The singer/actor shot to fame as Keith Partridge on the popular ABC sitcom in 1971, before embarking on a pop career.
Network bosses continually used his image to sell products branded with the hit series, but Cassidy only realised earlier this year (11) that ABC executives had raked in approximately $500 million (£312.5 million) from the merchandise - and he has been seeking a share of the profits ever since.
On Wednesday (05Oct11), Cassidy filed suit against Sony Pictures Entertainment bosses, who own the rights to the show, accusing them of fraud and breach of contract.
The legal papers, lodged in Los Angeles Superior Court, read: "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."
His legal team go on to state: "Mr. Cassidy has reason to believe, and does reasonably believe, that defendants have been perpetrating a scam."
Cassidy, 61, is demanding "in excess of millions of dollars" and he claims he has so far only received an estimated $5,000 (£3,125) from The Partridge Family merchandise.
He tells CNN, "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."
Under the terms of his original TV contract, Cassidy is reportedly entitled to 15 per cent of net merchandising revenues for the use of his likeness and a cut of profits from other items related to the show.
The singer/actor shot to fame as Keith Partridge on the popular programme and went on to establish himself as a pop star.
But Cassidy admits he had no idea how much money ABC network chiefs had made from the sale of products branded with the sitcom until recently - and the star, now 60, is in negotiations to cash in on the success himself.
He tells American breakfast show Today, "I was told after the show was over, The Partridge Family became the most successful financially because of merchandising and I had no part in it. They owed me a lot and I never received it, but they're making an effort to compensate me a little bit."