Third seed Halep won 6-4 3-6 6-3 to set up a final against unseeded Jelena Ostapenko of Latvia.
Czech second seed Pliskova would have become world number one next week had she won the second semi-final.
Ostapenko celebrated her 20th birthday with a 7-6 (7-4) 3-6 6-3 win over Switzerland's Timea Bacsinszky.
Mimi Jausovec was the last unseeded player to reach the French Open final in 1983, losing to Chris Evert.
Saturday's final will see a new Grand Slam champion crowned, and a new name at the top of the rankings should Halep win and overtake Angelique Kerber.
"It is nice to be in the final again," said Halep, who lost to Maria Sharapova in the 2014 French Open final.
"I hope I can play better and win it. I'm playing a young player - it is a big challenge."
After recovering from a set and 5-1 down against Elina Svitolina in the quarter-final, Halep led from the front against Pliskova.
With the number-one ranking and the prospect of a first Grand Slam title on the line, it was Halep who grasped the opportunity.
The 25-year-old broke serve in game three as her speed and accuracy proved too much for the wayward Pliskova.
The Czech made 24 errors in losing the first set, but finally edged ahead at 4-2 in the second.
Pliskova, 25, had said throughout the tournament that she was surprised at her progress on the clay, but for half an hour her powerful hitting held sway and there was little Halep could do.
The Romanian was under pressure early in the decider but finally chased down a forehand and fired a brilliant cross-court winner to grab a 3-1 lead.
Pliskova would cut the deficit to 4-3 only to blaze a forehand long, leaving Halep to serve out the match after two hours.
Ostapenko was well into uncharted territory, ranked 47th and having never been past the third round at a Grand Slam before, but she relished the big stage.
The former Wimbledon junior champion held her nerve to become Latvia's first major finalist, and the youngest woman in a Slam final since 19-year-old Caroline Wozniacki at the 2009 US Open.
Ostapenko edged a first-set tie-break but looked to have lost her way in the second when she dropped four games in a row.
Bacsinszky was knocked out of the tournament at the semi-final stage at Roland Garros in 2015 and appeared determined to avoid the same fate as she comfortably levelled at one set all.
But Ostapenko turned on the style in the decider, edging ahead at 4-3 with the 15th break of the match before holding for 5-3.
Ostapenko then hit a winning forehand to secure her place in the final, prompting the crowd to sing "happy birthday" as she celebrated the biggest win of her career so far.
"I felt a little bit tight because it was a semi-final," she said.
"But then I was just trying to be calm and just to try to play my game and just enjoy every moment."
Romania's Simona Halep moved within one win of a first Grand Slam title and the number one ranking with victory over Karolina Pliskova at the French Open.