The Ukrainian took nearly three hours to prevail 6-4 6-7 (3-7) 6-2 and win her first WTA 1000 title since lifting the title in Rome in 2018, having also won at the Foro Italico in 2017.
"It's very hard to believe that it's been eight years when I had this trophy here and very, very, very pleased of course with the two weeks here," the 31-year-old said on court after her 20th career title on Saturday.
"I told my coach that it would be nice before I finish my career to have a round number. So he told me that hopefully we're going to get this this year. So, I'm very, very happy."
Gauff, who was hoping to become the first American to win in Rome since 2016, lost to Jasmine Paolini last year but went on to win the French Open.
"It's been a great two weeks ... and I definitely feel the momentum going into the French Open," said the American. "A lot of lessons learned from this match."
Gauff started brilliantly, breaking Svitolina and holding to love. She also had the chance to go 5-2 up but the Ukrainian fended off three break points and won four-straight games to take the opener.
The third-ranked Gauff struggled on her serve throughout the second set but managed to take it to a tiebreak and level the match.
However, Svitolina dominated the decider and broke the American's serve twice before sealing victory on her third match point with a volley at the net.
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