The incident happened on Station Road, #KingsHeath at 3.20pm on Sunday 27 September.
A group of four youths ran up to the teenager and one punched him in the face. As he fell to the floor the group surrounded him and grabbed his mobile before heading off towards Kings Heath High Street.
Anyone with any information can contact Officers via Live Chat at www.west.midlands.police.uk, 8am to midnight, or call 101 anytime.
To tell Police what you know, not who you are, call Crimestoppers anonymously on 0800 555 111. Please quote crime number 20BE/234093V/20.