WASHINGTON — The Supreme Court sought to crack down on insider trading Tuesday, ruling unanimously that tips passed between relatives and friends are illegal even if the corporate insider receives no financial benefit.

The decision marked the first time the high court had clarified what constitutes insider trading in nearly two decades, and it upended a legal standard set by a New York-based federal appeals court in 2014 that had made prosecutions more difficult. The decision was written by Justice Samuel Alito.

“Giving a gift of trading information is the same thing as trading by the tipper followed by a gift of the proceeds,” Alito wrote.

Wall Street has been watching the case carefully for a sign of where the justices stand on the issue. The earlier…