Maria Sharapova’s doping suspension was reduced. Credit Aaron Favila/Associated Press

Maria Sharapova, the Russian tennis star, won a partial victory in the world’s top sports court on Tuesday after appealing her two-year suspension for a doping violation.

Sharapova’s punishment will be reduced by nine months, the Court of Arbitration for Sport announced, ruling that the original penalty was disproportionately harsh for an infraction that Sharapova committed unintentionally.

Sharapova, 29, tested positive for meldonium, a heart medication newly banned by antidoping regulators, at the Australian Open in January. She publicly disclosed the violation in March, days after she was privately notified of the lab results.

She will now be eligible to return to international competition on April 25, 2017, a week after her 30th birthday and in time for the French Open and Wimbledon.

Under the previous ruling, she would have been suspended until Jan. 25, 2018.

Tuesday’s ruling may help Sharapova redeem her standing with sponsors that quickly distanced themselves from her when she was disciplined last spring.

Some companies, including Nike, expressed support for Sharapova, a five-time Grand Slam champion, while others, like Porsche, said they would wait until the court delivered its ruling to decide whether to continue their relationship with her.

“I’ve gone from one of the toughest days of my career,” Sharapova said in a statement Tuesday, alluding to her initial suspension, “to, now, one of the happiest days, as I found out I can return to tennis in April.”

Meldonium, which is believed to improve blood flow and accelerate an athlete’s recovery while training, was not banned by the World Anti-Doping Agency until 2016. Sharapova has said she was unaware that use of the drug in international sports had been restricted.

She had taken the substance since 2006, she said, to treat an array of health problems, including frequent bouts of the flu, the possible onset of diabetes and a magnesium deficiency.

She said she failed to read an email from the antidoping agency in 2015 indicating that the drug would be prohibited the following year.

In June, a disciplinary panel appointed by the International Tennis Federation accepted Sharapova’s claim that the violation had been accidental, a finding that spared her from a suspension of up to four years.

Imposing the original two-year punishment, the panel said she bore “very significant fault” and “sole responsibility” for the positive test. (Her manager, who typically monitored changes in prohibited substances, had also been unaware of the medication’s changed status.)

Appealing to the sports arbitration court — the final authority on global sports disputes — Sharapova argued at a hearing in New York last month that tennis officials had not done enough to publicize the change and that two years was a disproportionately harsh penalty.

“This was an honest administrative mistake,” John Haggerty, a lawyer for Ms. Sharapova, said. “There was no intent to cheat.”

A panel of three arbitrators — two from the United States and one from Italy — accepted that explanation, but noted in a 28-page decision that even though the offense was inadvertent, it nonetheless constituted a violation of the rules that warranted a 15-month suspension.

The meldonium ban has resulted in a rash of failed drug tests among Eastern European athletes this year, predominantly Russians.

Russian coaches and sports officials have said taking meldonium is a common and longstanding method to increase energy and stamina, disputing that the substance enhances athletic performance. Some experts, too, have disagreed with the notion that it might deliver an unfair advantage, though effects of the drug on athletes are hazy.

The Russian tennis federation said this year that it had been unaware that Sharapova used meldonium, which she said she obtained in Russia. (The substance is not approved by the Food and Drug Administration in the United States, where Sharapova has been based for more than two decades.)

According to the International Tennis Federation, Sharapova said she informed the Russian Olympic team’s doctor in 2015 that she was taking the drug. She had not, however, disclosed her chronic use of it to antidoping authorities, who routinely request that athletes identify all medications they have recently taken when providing samples for testing.

Few people in her entourage other than her manager knew she took the medication, Sharapova said. Her selective disclosures, the tennis disciplinary panel wrote last summer, “must inevitably lead to the conclusion” that she took meldonium to enhance her performance; Sharapova has rejected that notion, asserting that the drug was simply a method to protect her health.

“There’s no scientific evidence that this is a substance that should even be on the prohibited list,” Mr. Haggerty said Tuesday, noting that point had been central to Sharapova’s appeal.

Although the tennis federation delivered a stiff punishment in June, the panel chose to backdate Sharapova’s suspension to the date of her initial infraction, in January, citing her cooperation and swift admission of a mistake.

Still, Sharapova — fighting to keep her sponsors and eager to return to the professional circuit — said she would not accept the “unfairly harsh” penalty.

In her statement Tuesday, Sharapova indirectly criticized the tennis federation for not telegraphing the change in rules, saying she had “learned how much better other federations were at notifying their athletes of the rule change, especially in Eastern Europe.”

She added: “I hope the ITF and other relevant tennis antidoping authorities will study what these other federations did, so that no other tennis player will have to go through what I went through.”

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