The Wimbledon champion pulled through 6-3 2-6 6-3 to cement her rise back up to sixth in the world rankings.
Halep, 30, dropped from second in the world to 20th after missing a large part of last season with injury.
Spain’s Pablo Carreño Busta beat Poland’s Hubert Hurkacz 3-6 6-3 6-3 to win the men’s title.
The 31-year-old pulled off what he described as the “best title of his career” after seeing off Britain’s Dan Evans in the semi-finals.
Carreño Busta, 23rd in the world, battled back after losing the first set on a single break of serve to turn the tables on world number 10 Hurkacz for a deserved victory.
Haddad Maia will have been dreaming of a similar upset against Halep, but will have to be content with breaking into the world’s top 20 after her run to the womein’s final in Toronto.
Two-time Grand Slam champion Halep showed all her resolve to record a 38th win of the season – only world number one Iga Swiatek has won more with 49.
She was broken in the opening game after serving four double faults, but from 3-2 down rattled off four consecutive games to win the opening set.
Haddad Maia went into the final having lost two of her three previous matches against the Romanian, but won their last encounter in three sets in the semi-final at Birmingham, on her way to the title.
And the 26-year-old ensured a decider in Toronto after racing into a four-game lead at the start of the second set before holding that advantage to level the match.
Halep has looked vulnerable at times this season, suffering a panic attack on court during a second-round defeat in the French Open, but also showed some fine form en route to the final in Toronto, winning one title in Melbourne in January and also reaching four other semi-finals – most notably at Wimbledon.
She traded breaks at the start of the final set before producing some of her best form of the season to make a decisive second break to lead 3-1 and was able to serve the match out, cementing her place among the favourites for the US Open title later this month.
The doubles finals follow, with GB’s Evans facing off against compatriot Neal Skupski, while French Open runner-up Coco Gauff is in action in the women’s.