Kourtney Kardashian is firing back at speculation she's a grandma ... blasting trolls online for spreading a rumor that her 15-year-old son is a teen father!

The reality television star jumped on Instagram Sunday to address internet speculation that her son, Mason Disick, has a kid of his own ... saying she doesn't usually address insane gossip like this, but felt she needed to step in for her child's sake.

KK straight up denies the rumor ... calling it a conspiracy and claiming many social media accounts pretending to be Mason are fake -- and, they're spreading lies about him.

Kourtney adds that Mason likes his privacy ... and, she's asking everyone to be respectful of it -- before calling on trolls to stop making videos about her minor kids.

ICYMI ... rumors about Mason allegedly having a 1-year-old child sparked when someone posted -- then deleted -- screengrabs on Reddit from a private Instagram account they said belonged to Mason.

While some ran with the false narrative, many questioned it from the beginning ... claiming it'd be hard for a Kardashian to keep a kid secret for more than year given the amount of attention the family attracts