She beat Latvia’s Jelena Ostapenko in Beijing on Saturday in the women’s singles semi-final.
The Romanian avenged this year’s French Open final defeat to the hard-hitting Ostapenko with a ruthless display of all-round tennis.
It saw her send down five aces and 14 winners to oust the 20-year-old in 75 minutes.
The 26-year-old takes over the top spot from Spaniard Garbine Muguruza and will become the first Romanian to head the rankings when the updated standings are released on Monday.
“It’s very emotional, I think it’s the first time I cried on court,” Halep said in a post-match interview.
“It’s amazing that I could do this. My team, everyone at home… is watching, and I want to thank everyone. It’s my special day.”
Ostapenko, for her part, put on a lacklustre performance and recorded 32 unforced errors as she failed to make any inroad on her opponent’s solid defence.
Halep also broke the Latvian’s serve four times, including in the first game of the match.
The Latvian showed some resistance in the second set to forge a 4-3 lead but Halep won three consecutive games to seal a victory she will forever remember.
WTA Chief Executive Officer (CEO) Steve Simon and tour president Micky Lawler presented Halep with a memento to mark her achievement after the match.
She had come close to topping the rankings on three different occasions earlier this year.
Halep will be looking for her second tournament victory of the season in what will be her fifth final of the year.
She can be up against either 12th-seeded Petra Kvitova of the Czech Republic or unseeded Frenchwoman Caroline Garcia on Sunday.