Mir Hazar Khan Khoso took the oath from President Asif Ali Zardari after the Election Commission selected him over three other candidates, Geo News reported.
Khoso will now have to call new elections for Parliament after the government of former Prime Minister Raja Pervez Ashraf was dissolved on March 16.
Khoso is the former chief justice of the high court of Balochistan and was twice appointed governor of the province for brief periods. He retired as chief justice of the federal Sharia court in 1994.
Speaking to reporters later, Khoso said elections would be held on time and rejected a perception that he would remain as prime minister past his stipulated appointment period, Dawn News reported.
He said his Cabinet would be comprised of only 10 to 12 people.