The 21-year-old victor needed two hours and 13 minutes to beat his 18-year-old opponent 1-6, 6-4, 7-5, reports Efe news agency.
World number 84 Medvedev looked far more composed in the second, gaining his reward with the only break and taking a tense battle into a final set when De Minaur was unable to return a powerful serve.
Russian qualifier Daniil Medvedev fought off tigerish Australian youngster Alex de Minaur to win his first ATP Tour title at the Sydney International on Saturday.
But not before the young Russian endured the hometown favourite's captivating third-set comeback from 4-0 down to very almost pull the rug right out from under his feet. Medvedev struggled in the opening set as the 167th-ranked de Minaur grew in confidence and was willed on by a supportive home crowd.
But De Minaur proved too steady, winning 6-4 6-4 to become the youngest male semi-finalist at the event in almost 30 years.
In the third, he was made to endure six deuces to hold one particularly gruelling service game but still won 11 of the last 12 games and, after being denied two match points, converted the third.
-Fabio Fognini (ITA, 4) defeated Adrian Mannarino (FRA, 5), 6-7 (4/7), 7-6 (7/4) and 6-2.
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"Until 6-2, 3-1, Fabio was playing unbelievable, and me, I felt that I didn't show my best tennis", Medvedev said.
In the women's match - Ashleigh Barty has tipped her Sydney International conqueror Angelique Kerber as a serious Australian Open contender.
The 18-year-old defeated the Spaniard in straight sets 6-4 6-4 to set up a clash with France's Benoit Paire.
The former world No.1 was too strong for Barty, winning the final 6-4, 6-4 to give her a massive confidence boost for the year's opening Grand Slam in Melbourne.
And Barty was convinced by what she'd seen, and experienced first-hand, of Kerber's nine-match unbeaten streak. "I played good tennis", Kerber said.
The second seeded Mertens downed Romania's 57th-ranked Buzarnescu, 6-1, 4-6, 6-3 in a decider frequently interrupted by rain.