Amplats said three weeks of illegal strikes by 28,000 workers in Rustenburg had cost it 700m rand ($82m; £51m) in revenue.
South African mining has been hit by a wave of wildcat strikes in which miners and officials have been killed.
Thirty-four platinum miners were shot dead by police on 16 August.
A separate strike is continuing at another mining firm, GoldFields, which is the world’s fourth-largest gold miner.
On Tuesday, GoldFields evicted 5,000 striking employees from company dormitories, saying they were intimidating fellow workers.
In all, about 75,000 miners are currently on strike in the gold and platinum sectors, most of them illegally, analysts say.
With unemployment in South Africa already at 25%, the mass dismissal will deal a blow both to the country’s weak economic growth and to President Jacob Zuma’s reputation as leader, says the BBC’s Milton Nkosi in Johannesburg.
The governing ANC party is holding a leadership contest in December, and some members are already calling for Mr Zuma to be replaced by his deputy, Kgalema Motlanthe.
Explaining its decision on Friday, Amplats said the miners had failed to attend disciplinary hearings and had therefore been dismissed.
Attendance levels of less than 20% meant four of the company’s mining operations in Rustenburg could not operate properly.
Employees would learn the outcome of disciplinary hearings later on Friday, and would have three days to appeal over their outcome, said the company.
“Approximately 12,000 striking employees chose not to make representations, nor attend the hearings, and have therefore been dismissed in their absence,” it added.
Amplats’ chief executive Chris Griffith said the company was still committed to “exploring the possibility of bringing forward wage negotiations within our current agreements”.
The ANC Youth League said it was “deeply disturbed and angered by the irrational and illogical firing”.
“This action demonstrates the insensibility and insensitivity of the company… which has made astronomical profits on the blood, sweat and tears of the very same workers that today the company can just fire with impunity,” said the league, which this week said it was backing Mr Motlanthe against President Zuma in the ANC contest.
“Amplats is a disgrace and a disappointment to the country at large, a representation of white monopoly capital out of touch and uncaring of the plight of the poor.”
The league pledged solidarity with the dismissed workers and called upon “all progressive forces” to support the call for their immediate return.
In another development on Friday, a trade union leader was shot dead at Marikana, near the volatile Lonmin platinum mine where weeks of violent protests left more than 40 people dead.
National Union of Mineworkers (NUM) spokesman Lesiba Seshoka told Reuters the official had been killed “execution style” but gave no further details.
Workers at the Marikana mine returned to work last month after receiving pay rises far higher than the rate of inflation.
The incident came hours after another deadly shooting, this time at the Amplats mine, in which a man died during clashes between striking miners and police.
Brigadier Thulani Ngubane told the BBC’s Newsday programme that the death had nothing to with the police action to disperse about 200 protesters near Rustenburg.
A commission of inquiry into the deaths at the Marikana mine began earlier this week.