The former world number one, playing only her second tournament after more than six months out, punched the air in delight after winning her first title since clinching Olympic gold in Athens last year.
It was the 20th tournament success of her career.
"It's amazing," Henin-Hardenne said. "I never thought when I came to Charleston that I could win this tournament."
In a match of the highest standard, second seed Dementieva, runner-up at the French Open and U.S. Open in 2004, began aggressively, pinning the Belgian well behind the baseline as she took a 5-3 lead.
The Russian forced a set point in the 10th game but Henin-Hardenne's forehand found the edge of the line and, after holding on for 5-5, she broke Dementieva two games later to take the set.
The players traded breaks at the start of the second set, but the Belgian's confidence was up and the 2003 champion broke twice to lead 4-1.
Dementieva refused to give up, however, breaking back twice to level at 4-4, before Henin-Hardenne, unseeded here after slipping down the rankings to number 43, broke again to lead 5-4 and then clinched a dramatic victory on her third match point.
FEELING PRESSURE
"At 4-3, I could feel the pressure," Henin-Hardenne said. "But I am happy that I kept fighting at 4-4 and the fact that I could break her was very important for me."
When Henin-Hardenne clinched the Australian Open in January last year, it was her third success in four consecutive grand slams.
However, the Belgian was then struck down by a virus and although she returned as defending champion at the French Open, she went out in the second round.
Henin-Hardenne then produced an heroic effort to win Olympic gold in Athens but, after exiting in the fourth round at the U.S. Open, she did not play again for the rest of the year.
A knee injury kept her out of the Australian Open in January before she returned at last month's event in Miami, where she reached the quarter-finals.
"I have won 20 tournaments but this one is very special," added Henin-Hardenne.
"It's been very emotional for me today because I came from something really bad last year and I'm so proud of what I did this week."
Henin-Hardenne is set to rise to 22nd in the rankings, while Dementieva will remain as world number five.
MISSED OPPORTUNITIES
"I think it was a great match," Dementieva said. "I just missed some opportunities in the first set when I was 5-3. I didn't take a risk and was maybe not as aggressive as I was earlier.
"Maybe it was just a bit of luck here and there but I think she deserved it. It's never easy to come back from injury and she played unbelievably on the important points.
"I missed a good chance to lead 5-4 in the second set, I was 40-15 up, but I am very satisfied with the way I played this week."
Dementieva, who will lead Russia in their Fed Cup first-round tie against Italy next weekend, believes she is on track for another strong showing at the French Open.
She lost to compatriot Anastasia Myskina in last year's final.
"I think this tournament can help me a lot," she said. "I've got my confidence.
"I just need some rest and will do a mental preparation. I hope to be in the same situation as last year. If I do, I'm sure I will not be so nervous."
