The Austrian had been beaten by Rafael Nadal in his previous appearance in the Barcelona final in 2017 but after knocking out the 11-times French Open champion in the semi-finals, Thiem easily dispatched the 23-year-old Medvedev.
Thiem made a slow start as he trailed 3-0 but he then got into a groove as he produced a number of scorching winners, including a sublime drop shot, to win four successive games and go 4-3 up.
Medvedev struggled to respond and called on the physio to get some treatment on his shoulder but that intervention failed to get him going and Thiem easily served out for the set.
Thiem faced little resistance in the second, using his drop shots to devastating effect to wipe out seventh seed Medvedev.
The Austrian third seed, who triumphed at Indian Wells last month, sealed victory with a simple volley.
"It's such an honour to win this tournament, looking at all the names who have won it I'm very happy and proud that my name is also there now," Thiem, who prevented Nadal from his bid to win a record-extending 12th title in Barcelona, told reporters.
"I had some trouble in the beginning and going 0-3 down. I had some troubles at the beginning with the slice then it got better and that's the only thing you can do against him and I'm happy that I made it work."
(Reporting by Richard Martin, editing by Pritha Sarkar)
