The way to Rihanna's heart ... is through her lungs ... 'cause the pop star says she got a WHOLE LOTTA MARIJUANA for Valentine's Day ... and she's totally stoked.

Rihanna posted her V-Day haul on Instagram ... along with the message, "Roses are green! Somebody knows how to make me happy."

The question is ... who got her the gift???

We already know Chris Brown has a California weed card -- so he can buy for HIMSELF in CA -- but it's against the law to buy pot for somebody else.

Still, Rihanna doesn't seem concerned.