The International Tennis Integrity Agency announced on Friday the world number nine tested positive for the blood-boosting drug Roxadustat at the US Open and has been provisionally suspended.
Halep, who won the French Open in 2018 and Wimbledon the following year, is the highest-profile tennis player to fail a drugs test since Maria Sharapova six years ago.
The 31-year-old Romanian immediately issued a statement on her social media channels, saying: "Today begins the hardest match of my life: a fight for the truth.
"I have been notified that I have tested positive for a substance called Roxadustat in an extremely low quantity, which came as the biggest shock of my life.
"Throughout my whole career, the idea of cheating never even crossed my mind once, as it is totally against all the values I have been educated with.
"Facing such an unfair situation, I feel completely confused and betrayed. I will fight until the end to prove that I never knowingly took any prohibited substance and I have faith that, sooner or later, the truth will come out.
"It's not about the titles or money. It's about honour, and the love story I have developed with the game of tennis over the last 25 years."
Halep, who suffered a shock loss to Daria Snigur in the first round of the US Open, brought her season to an end last month following nasal surgery and was notified of her failed test on October 7.
The former world number one requested her B sample was analysed, which confirmed the finding of the A sample.
Roxadustat has been linked to doping in the past, with the United States Anti-Doping Agency retesting samples in 2016 after saying it had been tipped off about the anaemia drug's use by athletes.
Roxadustat increases the production of red blood cells, helping endurance, and is in the same category of banned substances as EPO.
Halep has returned to the top 10 this season following injury problems last year, reaching the semi-finals of Wimbledon and winning WTA Tour titles in Melbourne and Toronto.