Pirapan, leader of the United Thai Nation Party, will be taking over from Deputy PM Somsak Thepsutin.
As per the order released on Wednesday, Pirapan will supervise the Justice Ministry on behalf of the premier, though Srettha will oversee the Department of Special Investigation on his own.
Somsak, meanwhile, has been tasked with overseeing the Public Health Ministry on behalf of the premier, while Deputy PM Phumtham Wechayachai is tasked with acting on behalf of Srettha when he is absent.
Somsak, a core Pheu Thai member who previously held the Justice portfolio, had earlier slammed the Corrections Department for failing to clear public doubts about former PM Thaksin Shinawatra being put up in a luxury suite in the Police General Hospital instead of spending time behind bars.
Political observers believe the Justice Ministry may have been handed to a coalition partner to prevent public criticism that Pheu Thai was abusing its power to help its patriarch, Thaksin.