Lynne Marie Stewart, 'Pee-wee's Playhouse,' 'American Graffiti' Star Dead at 78

Lynne Marie Stewart, the actress best known for her roles in "American Graffiti,"  "Pee-wee’s Playhouse" and "It’s Always Sunny in Philadelphia," has died, TMZ has confirmed.

Bette Smith, Stewart's manager tells TMZ ... the actress died unexpectedly yesterday afternoon at her sister's California home. In December, doctors found a tumor between the liver and gall bladder. Prior to the diagnosis, Stewart wrapped Ben Stiller's upcoming film "The Dink" ... as well as kept working on "It's Always Sunny in Philadelphia."

"She was the most talented and funny and most honorable person I have ever known," Smith told us of Stewart.

Cassandra Peterson, known for her iconic “Elvira” character, broke the sad news on IG, writing ... "My heart is breaking upon hearing the news of my dear friend Lynne Stewart’s passing. One of the kindest, sweetest, funniest women who ever lived. The iconic Miss Yvonne of Pee-wee’s Playhouse: She’ll always be 'the most beautiful woman in Puppetland'. 💔"

We reached out to a rep for Stewart for more info ... so far, no word back.

As Cassandra noted, Stewart played Miss Yvonne, The Most Beautiful Woman in Puppetland, on "Pee-wee’s Playhouse" from 1986 to 1990 ... and she appeared in several other 'Pee-wee' titles, including "Pee-wee's Big Adventure" and "Big Top Pee-wee."

The actress also appeared in the iconic George Lucas film "American Graffiti" and the Arnold Schwarzenegger classic, "The Running Man."

Stewart’s more recent fan-favorite role was as Charlie Kelly’s mother Bonnie, in "It’s Always Sunny in Philadelphia."

She was 78.