The girl was walking along Ena Street when a man approached her and asked to use her phone to call his dad.
When the girl refused, the man pushed her over, took her phone - a gold-coloured Samsung Galaxy S7 with an orange/black case - and ran off in the direction of Boulevard.
The man was white, slim and around 5ft 8ins tall, with gingery fair hair and facial stubble.
He was wearing a grey zip-up jacket and dark blue jeans.
Witnesses to the robbery, which occurred at 8.15am on Tuesday, March 20 is asked to call Humberside Police on 101 quoting log number 79 of March 20.