SpongeBob's great grandmother (possibly Great Grandma SquarePants or Great Grandma BubbleBottom), referred to as "great-grammyma," is one of four possible great grandmothers of SpongeBob SquarePants.
Although she has never made a physical appearance in the show as of yet, Plankton pretends to be her in the episode "Gramma's Secret Recipe." For his disguise, he wears a yellow and red long dress, with clear glasses and white, poofy hair.
He walks over to SpongeBob's house and fools him into thinking that he is his great-grandmother.
Towards the end of the episode, Plankton's real grandma realizes and enters the Krusty Krab, where she drags out Plankton, disguising as her. SpongeBob notices her and thinks that his great grandmother caught Plankton stealing the formula.