Martincova, who was only invited to play at short notice as a replacement for Marketa Vondrousova, was a surprise finalist but the Czech player met her match against a player ranked 130 places higher than her.
Pliskova prevailed 6-2 6-4 in a rematch of the pair's US Open first round meeting last year, which the former world number one also won.
She said on Twitter: "We all have been through a difficult time full of fear and uncertainty.
"This trophy means a lot to me, because it symbolizes that the world is finally coming back to the right direction. Thank you for all your support!"