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Madison Beer and Justin Herbert shared a lightning quick interaction at the Los Angeles Chargers game Sunday ... sharing a kiss on the sideline before the game!

The pop star stood just off the field at SoFi Stadium Sunday ... hanging with a pair of friends when her footballin' BF came over to say hello.

Like we said, it's short and sweet ... Herbo plants a kiss on Beer's lips and shakes her friends' hands before kissing Beer again and jogging off.

Basically, Justin's in game mode ... but he's still taking his time to show his lady some love for coming out to support him. Gallery Launch Gallery Instagram

As you know ... these two were first linked romantically back in August -- and we've shown you photos of the two walking around Los Angeles, celebrating his win

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