Justin Timberlake -- Grizzlies Good Luck Charm? MY GRANNY!

The Memphis Grizzlies have a secret weapon.

It's not Marc Gasol. It's not Zach Randolph.

It's Justin Timberlake's grandmother ... the team's best good luck charm.

JT -- a minority owner of the Grizzlies -- brought his grandma to watch his hometown squad take on the Miami Heat ... and guess what? The plan worked. Memphis smashed on D-Wade and company 103-87.

Afterward, Timberlake posted the family pic and wrote, "NBD. Just flexin' real hard at this @memgrizz game with my Granny. She must be good luck... We won by 16. #GoGrizz #GoGranny."