Josh Allen was all in on his fiancée while out at a "Sinners" premiere party Monday -- supporting her endlessly at the big movie event ... and even planting a kiss on her too!

The Buffalo Bills superstar traveled to Mexico City to show love to his soon-to-be-wife as she promoted her new Michael B. Jordan flick ... and he played the role of doting partner extremely well.

The footballer -- who rocked an all-black 'fit for the occasion -- watched her shine on the red carpet from afar ... before he stole a kiss from her as she had to be escorted off to her next engagement.

It's, of course, a return of a favor of sorts ... as Steinfeld has traveled to many of Allen's games since they got together in the spring of 2023.

The two now seem to be destined for a lifetime of being by each other's sides at their biggest career moments -- as they're set to wed soon following their fall engagement.

It's all so cute ... you could say it makes ya wanna shout!