Maneskin's Damiano David faced a barrage of speculation after footage emerged of him online leaning over a table in the VIP area of the event in Rotterdam.
While such alleged backstage antics are par for the course for some rock stars, the video sparked fury in France.
Clement Beaune, the country's European affairs minister and one of President Emmanuel Macron's closest advisors, called for "transparency" as to whether David had taken drugs.
The winner's group had edged French singer Barbara Pravi into second place.
But organizers said in a statement late on Monday that the singer had taken a drug test and the results were negative.
"Following allegations of drug use in the Green Room of the Eurovision Song Contest Grand Final on Saturday 22 May, the European Broadcasting Union (EBU), as requested by the Italian delegation, has conducted a thorough review of the facts, including checking all available footage," the statement said.
"A drug test was also voluntarily undertaken earlier today by the lead singer of the band Maneskin which has returned a negative result seen by the EBU."
The European Broadcasting Union, a club of European public broadcasters that stages the Eurovision Song Contest each year, criticized the rumors that swirled for the past two days.
The EBU said it was "alarmed that inaccurate speculation leading to fake news has overshadowed the spirit and the outcome of the event and unfairly affected the band."
During a press conference following their win, David denied that he used drugs and said he bent over because another band member had broken a glass at their feet.
Maneskin are the first group to win the competition since 2006. They also gave Italy its first victory since 1990.
The United Kingdom finished last in the competition, with James Newman's Embers picking up no points.