Anglo American Platinum has signed a recognition agreement with the Association of Mineworkers and Construction Union, City Press reported on Sunday.
"We have full organisation rights and will now be able to implement formal structures and appoint shop stewards at Amplats," Amcu president Joseph Mathunjwa told the newspaper.
Amplats spokeswoman Mpumi Sithole said Amcu now represented 40 percent of its workers, who number just under 60,000.
National Union of Mineworkers (NUM) spokesman Lesiba Seshoka said he believed workers would eventually return to his once-majority union.
He claimed companies were siding with Amcu "to get back at the ANC".
"Those employers should just pray that we don't become the majority union, because we will make life very difficult for them."
Last month, platinum producer Lonmin denied manipulating union membership verification processes to benefit Amcu.
"Lonmin believes in freedom of association. There is no intimidation. We cannot decide who joins what union," spokeswoman Sue Vey said at the time.
Her remarks came after Congress of SA Trade Unions general secretary Zwelinzima Vavi condemned the management of Impala Platinum (Implats), Amplats and Lonmin for apparently giving Amcu preferential treatment as it sought members.
"[They] are manipulating the membership verification process and employing the age-old tactic of divide-and-rule," said Vavi.
Platinum mines in and around Rustenburg have been plagued by labour unrest since August.
Violent protests said to be linked to rivalry between the NUM and Amcu have claimed the lives of at least 44 people.