Having earlier embarked on a 15-match winning run, Cronulla Sharks are without a victory in four NRL matches.
Cronulla Sharks fell to their third successive defeat in a 12-6 loss at South Sydney Rabbitohs on Monday, raising concerns over their form heading into the NRL finals.
The second-placed Sharks are now without a win in four after the Rabbitohs - out of the running for a finals spot - registered rare back-to-back victories.
Cody Walker scored the hosts' opening try at ANZ Stadium, with Damien Cook's penalty goal sending the Rabbitohs in with an 8-0 lead at half-time.
A Wade Graham try after the interval helped pull the Sharks to within two points half an hour from time, but the Rabbitohs defended resolutely to keep their opponents at bay.
Joe Burgess then wrapped up the victory by touching down in the closing seconds, earning Souths a seventh win of the campaign.
The Sharks embarked on a 15-match winning run prior to a draw with Gold Coast Titans at the beginning of their current barren run.