BEIJING—After investigating investors and brokerages in connection with a bungled summer stock-market rescue totaling more than $200 billion, Beijing is now probing the rescuers.

Communist Party graft busters are investigating whether officials inside the China Securities Regulatory Commission used their knowledge of the rescue effort to enrich their friends or themselves, say agency officials familiar with the probe. In recent weeks, they have been taking officials, one by one, to a hotel close to the agency’s headquarters...