Police were called at about 3.30pm after the teenager was assaulted on a snicket off Stoney Lane in Lightcliffe, Halifax.
She was walking in the direction of Aysgarth Avenue when the man approached her and is reported to have hit her on the head.
The young girl was able to run onto Aysgarth Avenue where she approached a member of the public and asked for help.
The man ran off in the opposite direction.
-> Leeds man jailed for hit and run that killed 68-year-old Ilkley womanDetective Inspector Gary Stephenson said: “Our enquiries are ongoing to trace the male involved and I would appeal for anyone with information to contact police.
“We will have extra officers patrolling the area today to provide reassurance to the community and conducting house to house and CCTV enquiries.
“I would appeal for anyone who saw a male acting suspiciously in the area at around the time of the assault or anyone with information to contact police.”
Police have asked anyone with information to call them on 101 or via the 101 live chat facility, quoting crime reference 13190134663.