Henin, 25, had recently experienced bad form and pulled out of this week's Rome Masters, citing fatigue as the factor.
However, nobody expected what was to come from the Belgian during today's press conference, just two weeks before the French Open - a tournament she has won for the last three years.
"I thought long about this," Henin said, with tears in her eyes. "I started thinking about it late last year. I was at the end of the road. I leave with my head held high."
Winning 10 tournaments last year alone, alongside seven Grand Slams in four years, Henin insisted that she would not go back on her decision to retire.
"This is the end of a child's dream," she continued. "I have experienced everything I could have. I have lived completely for tennis."
"I am relieved and proud of what I achieved."
Many rated Henin as the greatest woman to play tennis in recent years. Although only 5'5" and weighing 9 stone, she was able to overcome it with a devastating one-handed backhand - a rare skill that was stronger than most of her opponents' double-handed efforts.
John McEnroe described her as "the Roger Federer of women's tennis".
Martina Navratilova, winner of 18 Grand Slams, went further in 2007, saying that "maybe the guys have 'the male Justine Henin', because she is just head and shoulders above everyone else right now".
But with a recent divorce from Pierre-Yves Hardenne, as well as a car accident involving her eldest brother in 2007, her personal life has caused her problems on and off the court.
Henin joins fellow Belgian Kim Clijsters, who also reached world number one, in retiring early from the sport. Clijsters, who called time on her career in 2007 aged 23, is now married and has become a mother, giving birth to Jada Ellie in late February.
Henin's announcement also means that she will not be defending her Olympic gold for women's singles in Beijing this summer.
It is widely believed that Henin will to continue her work with her tennis academy, Club Justine N1, which opened last November in Limelette, a French speaking part of Belgium.