Bertens, last year's runner-up, defeated Halep for her second title of the year, adding to her victory in St. Petersburg, Russia.
"I am really proud of this week. I played some good tennis,'' said Bertens, who next week will reach a career-high No. 4 ranking.
The seventh-ranked Dutch became the first woman to win the Madrid title without dropping a set. She had victories over three Grand Slam champions -- Jelena Ostapenko, Sloane Stephens and Petra Kvitova -- in the Spanish capital. Bertens lost to Kvitova in last year's final.
Halep, winner in Madrid in 2016 and 2017, lost the chance to take over the No. 1 ranking from Naomi Osaka.