Officers were alerted at around 12.15pm on March 6 following reports of an incident in West View.
Earlier in the day, around 3.10am, a 21-year-old man from Ribbleton was walking home when he was approached by five to six males.
The group threatened the victim with a knife before repeatedly punching and stamping on him.
A white iPhone 5, black ‘Storm’ wallet containing £135 and a black North Face cap were stolen during the assault.
The victim suffered a cut to the cheek before making off from the scene.
Detectives are appealing for information following the robbery.
Det Con Andrew Crook, of Lancashire Police, said: “We have launched an investigation after a robbery in Preston.
“A 21-year-old man was viciously attacked by a group of males close to West View.
“The offenders, described as wearing hooded clothing, then made off with a number of the items belonging to the victim.
“If you saw or heard anything, please call police immediately.”
Anyone with information can call 101 quoting crime reference SA1703201.
Alternatively CrimeStoppers can be contacted anonymously on 0800 555 111 or online at www.crimestoppers-uk.org.