The 24-year-old hooper shared the kind gesture on IG Monday ... saying, "today I surprised them with a New House I always dreamed about living in myself, but I’m taking care of them first because without them, there’s no Alize."

In the clip, Johnson's family walks up to the new crib in their hometown of Williamsport, PA ... and his mother immediately breaking down when she realizes what's going on.

She falls to her knees, "Are you kidding me?!?" -- and Johnson has to literally carry her to the new home.

Amazing!!

Johnson explained his decision for the incredible gesture ... saying, "My family is the reason I go so hard. "The reason I sacrifice everyday and continue to put in work. I just want to see a smile on their faces."

This house is beautiful and the perfect size for his large family -- after all, Johnson is the oldest son of 7 SIBLINGS (4 sisters, 3 brothers).

Pacers star Victor Oladipo praised his teammate for the gesture, "Proud of you slim!! You know what time it is!!! Congrats to the Johnson Family!"

Johnson -- who will become a restricted free agent this off-season -- signed a 2-year contract worth $2.2 million with the Pacers in 2018.