When you're about to get one of the richest contracts in sports history ... what do you get your wife for her birthday?
Bryce Harper got his wife THE BACKSTREET BOYS!!
The MLB superstar took his wife to see the guys at Planet Hollywood in Vegas over the weekend -- and not only got her backstage for a meet and greet ... but had the guys take her on stage for a 5-on-1 serenade!
Check out the video as Kayla Harper gets a flower and a special rendition of "Shape of My Heart" from AJ, Howie, Nick, Kevin and Brian!