Sydney Sweeney's a boss ... whether making deals, making movies or even her making her way across the street with Scooter Braun -- just check out her outfit for all the proof ya need!

The film and TV star was spotted walking into a New York City office building Sunday afternoon ... showing off her long legs in a gray skirt with a matching gray top over a long-sleeve shirt.

It's a killer outfit for the boardroom ... though it seems she's spending the day with her better half instead of taking meetings.

Scooter looked relaxed in a black jacket and brown slacks ... gripping tightly to Sydney before disappearing into the building.

News of Scooter and Sydney's romance broke last month ... though our sources initially told us this was just a casual fling between the two of them.

We've seen them at dinners together ... and Scooter also hung out with Sydney and her parents at Universal Studios' Halloween Horror Nights -- which seems to indicate these two are in the midst of a very real relationship.

Braun made an appearance at Sydney's space-themed 28th birthday party -- during which Sweeney stunned in a busty metallic ensemble.

Like we said, our sources say this relationship is just casual ... but, Sydney's looking all business here!