The former One Direction singer, 26, was confronted by the mugger during a night out in Hampstead, according to the Mirror Online.
Scotland Yard confirmed the force is investigating a knifepoint robbery in the area.
A statement said: "Officers were contacted on Saturday February 15 regarding the incident which happened at 11.50pm on Friday February 14.
"It was reported that a man in his 20s was approached by another man and threatened him with a knife.
"The victim was not injured however, cash was taken from him. No arrests and enquiries are ongoing."
Styles is due to perform at the Brit Awards on Tuesday evening, where he is up for two gongs in the male solo artist and best album categories.
Last year, homeless Pablo Tarazaga-Orero, 26, was banned from going within 250 metres of the star after he was convicted of stalking, having spent months camped outside his house.