The 16-year-old played a game of touch football with the two-year-old, who seemed to be having the time of his life.
PHOTOS: Kendall Jenner Plays With Nephew Mason At The Beach
Although the football was almost as big as he, little Mason had the advantage because stick thin Kendall never let go of her smartphone!
Kendall was on baby-sitting duty, helping out her big sis Kourtney Kardashian who just had her second child with boyfriend Scott Disick.
Their daughter Penelope was born July 8.