The singer and actress said she had "no choice" but to leave the Channel 4 reality ski show because of a ruptured ligament.
Former England rugby player Ben Cohen, 37, who was drafted in as some of the stars were forced to drop out, was also injured after he said he used his face to stop himself in an accident.
He lost two teeth and needed 20 stitches, he said on Twitter, but will remain in the competition.
Harding, 34, said she was proud of her progress and thankful for the "once in a lifetime journey".
After revealing her injury, she tweeted that she was "gutted" to receive the news that after six weeks of training she would not be able to continue.
But she said to make it as far as she did had surpassed her own expectations "despite the ups and downs".
The show's third series has been plagued by injuries and has now seen six stars retire from the slopes.
Britain's most successful gymnast, Beth Tweddle, 30, needed neck surgery following a fall during training.
Former Olympic champion swimmer Rebecca Adlington, 26, withdrew from the show on medical advice after a shoulder injury.
She told host Davina McCall the pain of the fall was ''literally the worst thing that has ever happened to me, it was worse than childbirth''.
Holby City actress Tina Hobley, 44, also headed for the exit after she dislocated her elbow and suffered two fractures to her arm.
Made In Chelsea star Mark-Francis Vandelli, 26, pulled out after fracturing his ankle and Olympic gold-winning medallist Linford Christie became the fifth celebrity to pull out after he failed to recover in time from a hamstring injury.