The UMWA said late last week it was voted in during an organizing election to represent 444 workers as collective bargaining agent.
"We are very pleased to welcome these miners into the UMWA family," UMWA president Cecil Roberts said.
"These courageous miners sought us out in their struggle to gain a strong voice at work. We look forward to sitting down with the company to negotiate a fair and equitable collective bargaining agreement as soon as possible."
Peabody did not issue a public statement on the mine’s decision.
The underground mine in Equality, Illinois, has more than 20 million tons of recoverable coal reserves. It began shipping coal in 2002 and sold 3.5Mt in 2009.
The UMWA represents over 105,000 active and retired coal miners as well as manufacturing workers, public service employees, health care workers and professional employees in the US and Canada.