WASHINGTON—The Supreme Court handed the government a significant win Tuesday in its pursuit of insider trading, ruling prosecutors in such cases don’t always have to show that something valuable changed hands to prove a crime was committed.

The unanimous opinion restores some of the power the government lost in a 2014 federal court case. That two-year-old decision had cast doubt on just what constituted insider trading, an often-blurry concept, and forced law-enforcement officials to drop a number of high-profile cases including...