The U.S. Supreme Court overturned Marconi's patent in favor of Tesla in 1943. More precisely, the Court affirmed that broad claims in Marconi's U.S. wireless-telegraphy patent were invalid because earlier patents by Nikola Tesla, Oliver Lodge, and John Stone had already disclosed key elements.
The case was Marconi Wireless Telegraph Co. of America v. United States, decided on June 21, 1943. It arose from the Marconi company's attempt to obtain compensation from the U.S. government for wartime use of radio patents, rather than from Tesla's 1915 lawsuit against Marconi, which went nowhere.
The ruling did not declare Tesla the sole inventor of radio or erase Marconi's achievements. The Court specifically left Marconi's reputation as a successful pioneer of radio transmission outside the issue before it. It focused on whether particular patented improvements were novel, and found that earlier work by Tesla, Lodge, and Stone anticipated them.
A common mix-up is treating the decision as a simple Tesla-versus-Marconi victory. It was a patent-validity ruling involving several inventors, but 1943 is the correct year associated with the Supreme Court decision.