Seth Rogen Says Jack in the Box Pineapple Express Ad Not Related to Film

Jack in the Box is getting ready for 4/20 with a little help from a strain of weed that Seth Rogen made famous -- but they don't seem to have run it by him ... 'cause he's pissed.

Seth fired away Sunday after JITB published an ad earlier in the week promoting a new milkshake they're rolling out ahead of Wednesday's herb-filled holiday ... in which they explicitly dub their beverage as Pineapple Express shakes.

The ad also touts their love of "trees" ... and their pricing's at $4.20. So yeah, they're all in on the toke joke -- something the fast food chain has leaned into hard in recent years.

Anyway, Seth responded to their digital commercial, writing ... "This has nothing to do with the movie Pineapple Express. We had nothing to do with this and weren’t asked if we wanted our film associated with this."

He adds, "And obviously if we did a tie in, it would have to be to promote those bizarre f***ing diarrhea inducing taco things they sell."

Of course, Seth is referring to his 2008 cult comedy classic of the same name -- which (fun fact) actually spurred the real-life weed strain ... at least according to Seth and co. Apparently, it didn't exist before they came along with their made-up bud ... so they do seem to have a claim to it, at least colloquially.

On the legal end, it's unclear if Seth has "Pineapple Express" trademarked or copyrighted. The fact he isn't demanding they yank the ad is kinda telling -- ya gotta figure if he really didn't want the name associated with Jack in the Box, he'd invoke different verbiage.