The answer was easy, Justice Alito wrote.

“Maher would have breached his duty had he personally traded on the information here himself then given the proceeds as a gift to his brother,” Justice Alito wrote. “It is obvious that Maher would personally benefit in that situation. But Maher effectively achieved the same result by disclosing the information to Michael, and allowing him to trade on it.”

Mr. Salman had relied on a 2014 decision from the United States Court of Appeals for the Second Circuit, in Manhattan, United States v. Newman, that made it harder to prosecute insider trading cases. That decision had dealt a setback to Preet Bharara, the United States attorney in Manhattan, whose office oversaw a sweeping crackdown on insider trading in the $3 trillion hedge fund industry.

Mr. Bharara applauded the Supreme Court’s decision.

“The court stood up for common sense and affirmed what we have been arguing from the outset — that the law absolutely prohibits insiders from advantaging their friends and relatives at the expense of the trading public,” he said in a statement. “Today’s decision is a victory for fair markets and those who believe that the system should not be rigged.”

In Mr. Salman’s case, the United States Court of Appeals for the Ninth Circuit, in San Francisco, rejected the reasoning in the Newman case. The Supreme Court on Tuesday affirmed that decision.