Two terrified youngsters were threatened with a knife near the travelling fair in Skelmersdale.
The teenagers – a 15-year-old girl and a 18-year-old boy – were approached by a man with a knife near the Asda store.
The man, who is said to have had a local accent, asked for their mobile phones.
They handed over the phones and the thief handed one back before making off with the second in the direction of Northway.
The second phone was later found discarded in Inglewhite near the park area.
Both phones were iPhone 5Cs.
The robber is described as white, aged between 19 and 21, 6ft and slim.
He had a pale complexion and a slim, long face and close shaven, light brown hair.
At the time of the offence he was wearing a royal blue pullover hoody with large white block writing on the bottom half, black tracksuit bottoms and Adidas ZX 50 trainers in cream.
Lancashire police are appealing for information.
DC Kate Ellison, of South CID at Skelmersdale, said: “This must have been a very scary experience for these two youngsters and thankfully they were not hurt.” She added: “I would appeal for anyone with information or who recognises the description of the offender to get in touch.”
Anyone with information should contact Lancashire Police on 101 quoting log number 1184 of April 23.
Or alternatively you can call CrimeStoppers, anonymously, on 0800 555 111 or report a crime online via their website, www.crimestoppers- uk.org