De Kock was already the captain of South Africa's ODI and T20I teams.
De Kock succeeds Faf du Plessis, who stepped down as a captain following South Africa’s 3-1 Test series defeat at the hands of England at home earlier this year.
CSA director of cricket, Graeme Smith, had initially ruled De Kock out of the Test leadership because the latter already has enough on his plate as white-ball captain, key batsman and wicketkeeper.
South Africa are slated to play seven Tests this summer – two against Sri Lanka, two in Pakistan and three against Australia.
CSA also announced a 15-man squad to face Sri Lanka, which includes maiden call-ups for Sarel Erwee, Kyle Verreynne and Glenton Stuurman.
Kagiso Rabada and Dwaine Pretorius, who sustained a groin and hamstring injury respectively during the recent white-ball series against England, haven’t been named in the squad at this stage. Both of them could be added in the coming days as their recovery from injuries is being assessed.
South Africa’s Squad for Sri Lanka Tests:
Quinton de Kock (capt, wk), Temba Bavuma, Aiden Markram, Faf du Plessis, Beuran Hendricks, Dean Elgar, Keshav Maharaj, Lungi Ngidi, Rassie van der Dussen, Sarel Erwee, Anrich Nortje, Glenton Stuurman, Wiaan Mulder, Keegan Petersen, Kyle Verreynne.