Halep came through a testing quarter-final clash with resurgent former world No 1 Angelique Kerber, coming from behind to win 6-7, 6-3, 6-2.
Halep took her place in a second consecutive Grand Slam final after losing in the final of the Australian Open in January to Caroline Wozniacki and fourth final at the majors overall.
However, her sights now turn to the final where she is desperate to erase the memories of last year's horror show where she surrendered a set and a 3-0 lead to lose to Jelena Ostapenko.
10th-seeded Sloane Stephens had never made it past the fourth round in Paris until now.
Backed by fans who loudly chanted her first name between games, Halep went ahead 3-0 with the help of only one victor.
It was 5-0 by the time Muguruza eventually claimed a game.
She has bounced back from disappointment after disappointment in the Grand Slam tournaments, where reputations and legacies are made and unmade.
Sharapova, playing at Roland Garros for the first time since 2015, suffered her most one-sided defeat at the Slams since a 6-3, 6-0 loss to Victoria Azarenka in the 2012 Australian Open final. "She had a lot more depth in the ball". She's clear on who she is as a player. I'm playing well, in a good space.
Sharapova make show on French Open, next rival Serena
Going by the history though, Serena has a 19-2 record advantage over Sharapova with her two losses coming way back in 2004. You know, I gave up so much, from time with my daughter to time with my family".
"I'm happy", she said, "that I can have another chance". You just have a feeling that the tennis gods are going to reward Halep with her first major, which is certainly overdo. Of her first 14 points, 13 arrived via errors by Muguruza - nine unforced, four forced.
A win at the Miami Open in April seemed to turn things around, but she followed that by failing to advance past the third round in each of her next four tournaments. Some of this is X's and O's; Sloane makes you hit the extra ball and has played terrific tennis in the matches, and Keys has missed too much.
"Another great opportunity on Saturday and I'm really looking forward to it", said Stephens, whose late father, John, was the 1988 NFL Offensive Rookie of the Year with the New England Patriots.
After racing through the first set, Halep saved three break points in a marathon ninth game of the second which lasted 13 minutes.
Halep converted her first match point when Muguruza sent a final backhand long.
He briefly lost his focus toward the end of the second set when he appeared to be distracted by a spectator just before hitting a double-fault to drop his serve and give Cilic a 5-4 lead.
"I have to expect some hard points, power".
There were no such fireworks this time, though, with Halep sealing a 6-7 (7-2) 6-3 6-2 victory. "I think my body will need a little bit of rest, but mentally I'm ready to go", said five-time major victor Sharapova.