Jesse Jane -- a well-known porn star from the 2000s -- has died alongside her boyfriend, who also passed away, and it sounds like their death is drug-related ... TMZ has learned.

Law enforcement sources tell us the famed adult film actress was found dead at a home Wednesday in Moore, OK -- where we're told police responded to this residence on a welfare check ... and when they got there, our sources say officers discovered both Jesse and her boyfriend, Brett Hasenmueller, deceased.

We're told the welfare check request was actually placed by Brett's employer, who hadn't heard from him in a few days. At the moment, we're told this appears to have been a drug overdose of some sort.

Unclear how long exactly Jesse and Brett might've been dead at the house, and an exact cause of death isn't known at this point either. An investigation is underway, and the Medical Examiner will make a final determination.

Jesse was one of the biggest porn stars in her day -- she started out with Digital Playground in 2002 ... and shortly thereafter, started to make appearances on Showtime's 'Family Business' -- and from there, her career in the industry took off.

She starred in a number of well-known adult films and adult film series -- including the 'Pirates' flicks ... notably the sequel in "Pirates II: Stagnetti's Revenge," which reportedly cost around $8 million to make and is considered one of the most expensive pornos to date.

Eventually, Jesse made her way over to Playboy ... serving as a host for a popular live show called 'Night Calls,' and even hosting Playboy TV's 'Naughty Amateur Home Videos.'

Jesse became a huge fixture in the porn world -- she hosted several award shows, including the AVN Awards multiple times, and she launched successful sex toy lines as well.

Outside of adult entertainment ... Jesse made some appearances in mainstream films as well, including 'Starsky & Hutch,' a guest role on 'Entourage,' and even an appearance on 'The Bad Girls Club.' In 2009, she was part of a CNBC doc called 'Porn: Business of Pleasure' -- which showed what her life was like behind the scenes, and away from cameras.

She survived by her one son. Jesse was 43.