The world number 44 dispatched the Romanian 6-3 6-0 in 64 minutes to reach her first final at a premier mandatory event which are the most prestigious on the tour after the four Grand Slam tournaments in Melbourne, Paris, London and New York.
Osaka beat the former world number one Maria Sharapova and then accounted for former top 10 player Agnieszka Radwanska.
"I just was not ready," said Halep after the mauling. "I missed the ball a lot and I didn't play what I had to play. I don't find excuses. She was better. I just was not ready and played wrong."
Osaka, 20, will play another 20-year-old in the final after Daria Kasatkina saw off Venus Williams 4-6 6-4 7-5.
"I feel like there is a new generation and we are trying to push through," said Osaka. "I was kind of nervous for that entire final game because it kept going back and forth. But I'm really glad I was able to finish on my serve instead of having to break her."
While Osaka accounted for the top seed, Kasatkina beat the second seed Caroline Wozniacki in the fourth round and followed that up with victory over the 10th seed Angelique Kerber before removing Williams
"It is maybe one of the best nights in my life, for sure," she said after recording her second win in three matches against Williams. "As my coach said: 'Williams is 37 years old and you are 20. You should try and win in three sets'."