Two female corrections officers have resigned after it emerged they were allegedly engaged in sexual relationships with inmates at a WA prison.

It is understood the two Serco-employed officers at Acacia Prison tendered their resignations last week following an internal investigation by the Department of Justice.

Acacia Prison.

"Serco can confirm two female staff members at Acacia Prison have resigned over alleged professional misconduct," a Serco spokesperson told WAtoday.

"Serco is investigating the allegations."