Stefon Diggs went all out for Cardi B 's big night ... gifting his girl a flower surprise so big, it had to be delivered on a flatbed truck!!

The touching gesture went down Thursday in New York City ... just hours before Cardi's "Am I the Drama?" was released -- her first album since 2018. Getty

The New England Patriots star was present for the reveal ... and judging by Cardi's reaction, she never saw it coming!!

The finished product was a MASSIVE custom container filled with roses from Venus Et Fleur ... which featured Cardi's face and LP name right on it.

Diggs -- who signed with the Pats in March -- has been very supportive of his girlfriend's second album ... writing in her Instagram comment section, "Proud of you for staying focused ❤️"

He's also excited about their new bun

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