De Kock scored his runs off 44 balls and shared an 79-run partnership with AB de Villiers as South Africa piled up a total of 195 for four after winning the toss.
Bangladesh fell short in their reply, finishing on 175-9 despite a bludgeoning start in which Soumya Sarkar struck 47 off 31 balls before a review had him trapped lbw to Andile Phehlukwayo, who finished with figures of 2-25 off his four overs.
There were two wickets on his international debut for Robbie Frylinck after a surprise call-up by South Africa for the 33-year-old bowler, including removing Bangladesh captain Shakib Al Hasan for 13.
The home triumph was built on the second-wicket partnership between De Kock and De Villiers, who added 49 off 27 balls.
Bangladesh threatened to make full use of perfect batting conditions as they started with purpose and were averaging more than 10 runs an over before wickets began to tumble and their run rate was pulled back.
Bangladesh complete their five-week tour with a second T20 international in Potchefstroom on Sunday.
They were comprehensively beaten in two tests and hammered by South Africa in the three one-day internationals.