Check out this footage showing Costa, in a white shirt, running on the stage Saturday night with mic in hand at the Ultra South Africa concert near Johannesburg -- he went right into one of his songs and seemed totally fine.

Then the video cuts to Costa belting out another tune later in the concert -- he seems to reach for one of his backup singers before tumbling to the ground. He got back up and finished the song, but soon fell down again.

His team and emergency responders attempted to resuscitate him, but they were unable to save him. His cause of death was not disclosed.

Costa's family released a statement thanking first responders ... while a rep for the Ultra South Africa venue says they're “devastated” by Costa's death and will remember him as a "talented rapper, dancer, singer, songwriter, collaborator and friend to the festival."

Costa was best known for his hit "Big Flexa," which has 45 million views on YouTube and featured Akon on the remix.

Akon, who signed Costa to his Konvict Muzik imprint, paid an emotional to his artist on IG ... praising him for being "hardworking" and noting he died doing what he loved the most.

"Costa Titch had a vision that I knew for sure would take over the music industry globally and was on his way to doing just that when god’s plan intervened, Akon wrote. "It’s been a sad day but what makes me feel better is that I know he died doing what he loved doing the most in this world. God bless you COSTA!!!!!"

Costa was 28.