A taxi driver suffered a fractured skull when he was hit over the head on Friday evening.
Police said the 45-year-old man was getting out of his taxi on Wake Road, Nether Edge, Sheffield, when he was struck on the head by an unknown suspect.
The weapon in the attack, which occurred at about 7.45pm, is unknown.
The taxi driver was taken to hospital but his injuries are thought to be non-life threatening.
A police spokesman said: “Police are investigating the circumstances surrounding this incident.
“Anyone who witnessed the incident is asked to call police on 101 quoting incident number 994 of January 30.
“Alternatively, to give information anonymously call Crime Stoppers on 0800 555 111.”