Thiem had lost all three of his previous matches against Klizan, but the top seed had no such trouble on Sunday as he eased to a 6-3 6-1 victory in just 68 minutes.
The 25-year-old Austrian faced just two break points in the match, saving both of them in the final game, and broke Klizan's serve on three occasions to end the Slovakian's perfect 6-0 record in finals.
Thiem, who leads the ATP World Tour with 48 match wins in 2018, is making a push to qualify for the Nitto ATP Finals for the third consecutive year and his first title on hard courts since 2016 is set to take him above Kevin Anderson into seventh place in the ATP Race To London.
In Metz, Gilles Simon delighted the home crowd by claiming the Moselle Open title for the third time in nine years, beating German qualifier Matthias Bachinger 7-6 (7/2) 6-1.
The 33-year-old Frenchman has now won two titles in the same season for the first time since 2011 and after starting the season 89th in the rankings, will return to the world's top 30 on Monday.
Despite the defeat, Bachinger enjoyed the best week of his career after reaching his maiden ATP World Tour final. The 31-year-old had recorded just one tour-level victory before the week but qualified and won four matches and will climb to 135th in the rankings.