One junk-bond trader at Goldman Sachs earned more than $100 million in trading profits for the firm earlier this year, an unusual gain at a time when new regulations have pushed Wall Street to take fewer risks.

The gains were the work of Tom Malafronte, a managing director on the bank's high-yield-bond desk in New York. The 34-year-old trader bought billions of dollars in junk corporate debt on the cheap starting in January, then locked in profits as prices recovered, according to people familiar with the matter.