The conviction of Jeffrey Davies, 45, at Newport Crown Court follows an investigation by the force, managed by the Independent Police Complaints Commission (IPCC).
The rapes happened in 2002 and 2003 while he was a serving officer in the Rhondda.
He was convicted on Thursday and will be sentenced at a later date.
Davies was dismissed from South Wales Police in 2013 after being convicted of other sexual assaults and the latest rape allegations followed publicity from those cases.
He was convicted of raping one woman in 2002, who he met through work and drove up to the Bwlch mountain in Rhondda Cynon Taff where he attacked her, and another woman who he raped in 2003.
Chief Supt Dorian Lloyd, head of South Wales Police's professional standards department, said: "He abused his position as a police officer and gave no regard to the traumatic effect that his actions would have on his victims.
"These were vulnerable women who, having encountered Davies, found themselves subjected to an appalling abuse of trust."
IPCC Commissioner for Wales, Jan Williams, said: "Jeffrey Davies was a sex offender hiding within the police service, who committed fundamental breaches of the trust placed in him as a serving officer."
A former South Wales Police detective has been found guilty of raping two women.