The victim was attacked on Tuesday morning, off Beta Road, Farnborough, Hants, police said. A police cordon has been set up around the scene.
Police said the 15-year-old was being questioned in connection with the woodland attack. No further details were released.
Police appealed for anyone in the area at about 0830 BST to come forward.
The 11-year-old boy is still being interviewed by specially trained officers who are trying to establish what happened.
Det Ch Insp Jim Antczak, of Hampshire Constabulary, said extra officers would be put on patrol in the area.
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