The 16-year-old was in woodland, between Howitts Lane and Howitts Gardens, in St Neots, at about 17:30 BST, when he came off his bike.
He suffered serious injuries to his neck and wind pipe, Cambridgeshire Police said.
He was rushed to Addenbrooke's Hospital where he remains in a serious but stable condition.
Det Con Diane Tomlinson said: "The young man is in a serious condition in hospital and I would like to speak with anyone who may know why the rope was tied to those trees, or who may have put it there.
"Anyone who uses the area, such as dog walkers, who may have information, no matter how insignificant it may seem, should call police."
A teenage motorcyclist was seriously injured when he came off his bike after hitting a rope tied between two trees.