The Romanian fourth seed, who will face 2012 champion and last year's runner-up Maria Sharapova on Saturday, had Petkovic under her spell in the opening set but had to fend off some spirited resistance in the second.
She fell a break down but recovered to force a tiebreak which she always controlled.
Halep, who has yet to drop a set in the tournament, ended the contest on her first match point with her 15th forehand winner.
"It's incredible I will play the final here in Paris," said Halep. "She played really well but I think I was a little bit stronger in the end.
"I have a lot of confidence in myself now. I played really well here. The next round will be tough. I played her (Sharapova) in Madrid. Why not take my revenge," she added.
Sharapova, who beat Canadian Eugenie Bouchard 4-6 7-5 6-2 in the other semi-final, has an 18-1 record on clay this season, including a three-set win over Halep in the Madrid final.
Injury-prone Petkovic, playing at the French Open for the first time since reaching the quarter-finals in 2011, said she had paid for a slow start.
"I'm very disappointed because I had a lot of chances in the second set," she said.
"I feel like the first set Simona played really great tennis, and I had trouble getting into my rhythm and trouble with my footwork. I wasn't really on fire."
Petkovic made six unforced errors in the first two games and the shrewd Halep, last year's most improved player on the women's tour, quickly opened a 5-1 lead on a sunny Philippe Chatrier court.
While not packing too much power, Halep compensates with beguiling anticipation and great footwork while her accuracy off the ground had Petkovic chasing left and right.
Once the nerves had settled Petkovic's greater firepower began to make an impression and she converted her first break point in the fourth game with a forehand winner.
Suddenly, it was Halep who was running around chasing balls.
The German showed signs of frustration in the following game, however, sending a routine forehand wide to concede the break and allow Halep to regain some control.
Halep, managed by compatriot Virginia Ruzici, the 1978 Roland Garros champion, took her chance and levelled for 3-3 and remained solid as the set went into a decider.
After opening a 4-1 lead, Halep kept her composure to become her country's first grand slam finalist since Ruzici in 1980 when she lost to Chris Evert.
(Reporting by Julien Pretot; Editing by Martyn Herman)
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