Police have arrested two men have been arrested for questioning over the collision near Mila’s Lake in Colin Glen Forest Park.
The woman was walking along a pathway near the lake when she was hit by the scrambler.
She was taken to the Royal Victoria Hospital by ambulance shortly after 5.45 pm and was due to undergo surgery later.
Sinn Fein west Belfast MLA Jennifer McCann said she believed the woman is a local mother with young children and expressed concern for the family.
She said the “thoughts and prayers” of the whole community in west Belfast were with the woman and her relatives.
“It’s really, really tragic that this has happened.”
Community workers have said it was not the first serious incident involving motorcycles and quads in Colin Glen Forest Park.