The Wimbledon champion, seeded four, claimed her second title of the season with surprising ease, as she swept aside the Romanian 6-1 6-0 in Cincinnati.
Halep knew victory would see her move to the top of the ranking above Karolina Pliskova, who was beaten by Muguruza in the semi-finals, but this defeat means she'll remain five points behind the Czech on Monday.
Muguruza is Sky Bet's 11/2 favourite to win next month's US Open while Pliskova is next in the running at 13/2 ahead of Angelique Kerber (15/2) and Johanna Konta (8/1). Halep is 9/1.
told the WTA website: "I think today was my best performance of the tournament, because I felt like I played 20 hours in that court, you know. I had very long matches. It was just there. I think I did few little mistakes and my shots were very [well] placed and I think I was doing almost everything right there.
"Since the first ball, honestly, I knew there was going to be a lot of long rallies, and she's a very solid player and, you know, very tough, as well. I was ready for it. I was looking to play my game, to be aggressive since the first moment."
The Spaniard was in total command during the 56-minute contest - as the scoreline emphatically suggests - although a key moment came in a marathon seven-deuce game in the second set when Halep failed to convert two break points as she desperately sought a way back into the match.
"I felt there was a key game in the second set, the 3-0 game, but I felt -- I felt very comfortable there, playing against Simona, and also my level was very high," added Muguruza. "I felt like especially in the second set I could raise it a little bit more and close the match."
Of Halep, she added: "I was thinking about [Halep reaching No.1]. I'm like, 'Oh, I wish I was in her situation.' It must be difficult, but I wanted the title, as well. Now you're telling me I'm going to be No.3, but I didn't know that. I feel like a lot of people are asking me about No. 1, stuff like that. All I want is to win the tournaments. That's what...motivates me always.
"[The ranking] is going to be a consequence. If I keep playing like this, sooner or later I guess I will be, I don't know. I hope so - but, no, it's not here yet like that."
Related Tennis Links
Sky Bet's Tennis Odds