The African National Congress, which was forced to do a deal with rival parties after getting less than half of the votes in the May 29 election for the first time in three decades, had been wrangling with the main opposition Democratic Alliance (DA) over who gets which cabinet posts.
"The announcement ... will be in the form of a televised address to the nation," a statement from presidency spokesperson Vincent Magwenya said, adding that it would be at 9 p.m. (1900 GMT).
The negotiations were the last hurdle to setting up a government. Two national newspapers reported a breakthrough on Sunday, after the DA backed down on a demand to take the trade and industry portfolio.
For decades the ANC had traded on its legacy of ending apartheid 30 years ago, when Nelson Mandela came to power and successfully reconciled a country deeply divided along racial, ethnic and class lines.
But voters became angered by the ANC's poor record on delivering on basic services including water, schools and electricity. It got just 40% of votes in the election, its lowest score ever.
Whether the current loose coalition of former enemies can do better may depend on the extent to which they can put aside their ideological differences, analysts say. There are several potential hurdles to policymaking.
The DA wants to scrap some of the ANC's Black empowerment programmes, saying that they have mostly made a politically-connected business elite fabulously wealthy while much of the Black majority stays poor.
It opposes the ANC's desire to expropriate land - most of which is in white hands as a legacy of conquest by colonists and subsequent entrenched white minority rule - without compensation and give it to Black farmers.
The DA also seeks to phrase out South Africa's minimum wage, currently sitting at 27.58 rand ($1.52) a hour, arguing it makes the workforce uncompetitive.
($1 = 18.1850 rand)
