Klaasen proved swashbuckling with the bat in both the one-day International and Twenty20 series against India over the last month and selector Linda Zondi said, "we firmly believe he can become just as good a player in the five-day format".
The 26-year-old has played four ODIs and two T20 matches for South Africa. "The selection of Mulder continues our strategy to expand our pool of all-rounders against top quality opposition," added Zondi in a Cricket South Africa statement.
"He is a batting all-rounder and his selection will depend on how we decide to balance our starting XI."
Mulder, 20, represented South Africa in an ODI against Bangladesh in October last year.
The duo take the places of Chris Morris and Andile Phehlukwayo in the squad that beat India 2-1 in their recent three-test series. The first test against Australia is in Durban, starting on Thursday with the second in Port Elizabeth from March 9-13.
Squad: Faf du Plessis (captain), Hashim Amla, Temba Bavuma, Quinton de Kock, Theunis de Bruyn, AB de Villiers, Dean Elgar, Heinrich Klaasen, Keshav Maharaj, Aiden Markram, Morne Morkel, Wiaan Mulder, Lungisani Ngidi, Vernon Philander, Kagiso Rabada.
First Published: February 24, 2018, 3:23 PM IST