With the win, Fernandez climbs five spots to No.22 in rankings and reclaims the Canadian No.1 spot from Victoria Mboko.
Playing in her eighth career final, the one-time US Open runner-up raced through the first set on Sunday in 29 minutes.
Valentova, yet to win a maiden WTA title, settled her nerves in the second set and found a way to level the match when she broke the fourth seed in the 12th game.
In the deciding set, the 27th-ranked Fernandez made a crucial break in the fourth game and then saw off a late challenge by the Czech to claim her second title of the season after taking the WTA 500 D.C. Open in July.
Fernandez won't have much time to rest since she is now set to compete at next week's tournament in Tokyo, where she plays compatriot Mboko in the first round.
"Congratulations Tereza. You played amazing. Wonderful week to you, your coach, and your family. I'm sure I'm gonna see you in many more finals like this one," 23-year-old Fernandez said during her trophy speech.
"Finally, I wanna thank my team. My dad who's over here who's also my coach, my hitting partner Federico ... thank you so much for suffering with me this week, this past month and past year.
"I also wanna thank my family and loved ones back home. Without you I wouldn't be here ... without everyone's sacrifices and support, I wouldn't be on this stage. Thank you so much for all the love and the motivation that you've given me."
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