Steve Jobs -- Iconic Portrait Was RIPPED OFF ... Says Photog

Steve Jobs was vigilant in protecting his copyrights ... and now the photographer who shot his most iconic portrait is following Steve's lead ... and suing two artists who allegedly ripped him off ... TMZ has learned.

The man behind the suit is Albert M. Watson -- a prolific photographer who's considered one of the most influential people to ever wield a camera. He shot Steve's famous portrait during a 2006 photo shoot ... and claims he owns the rights to the work.

FYI -- the shot was used as the cover art on Walter Isaacson's biography, "Steve Jobs."

But in his lawsuit, filed in federal court in NY, Watson claims two well known artists -- Alex Guofeng Cao and David Datuna -- used his famous "Steve Jobs" portrait as the basis for new pieces ... but they never got proper permission.

Watson claims one of the unauthorized works, "Steve Jobs vs. Neil Armstrong, After Watson," has become popular and is being sold in multiple galleries across the country.

He also claims a Datuna/Cao ripoff, titled "Steve Jobs-Ayn Rand" was sold to a private collector for $210,000 back in 2011.

In his suit, Watson is demanding a judge put a stop to Datuna and Cao's actions -- claiming his "rights, business, goodwill and reputation have been and will continue to be irreparably harmed" if they continue to jack and sell his stuff.

He's asking for unspecified damages.