They were approached by another man and a woman walking along the street. The man then punched one of the 17-year-olds in the face, fracturing his jaw in two places.
Police are appealing for anyone who witnessed the assault or knows who is responsible to contact them.
PC Mark Weir of Harrogate police who is investigating the incident, said:
This was a nasty, unprovoked attack on an innocent member of the public and we need to bring the person responsible to justice.
I am appealing for anyone who witnessed the assault or knows anything about it to contact me. In particular I urge the woman who was with the offender at the time, to do the right thing and come forward. The victim was seriously injured and she is a vital witness to the incident.
Anyone with any information is asked to contact North Yorkshire Police on 101, select option 2 and ask for Mark Weir. Or you can email him at email@example.com
If you prefer to remain anonymous, you can ring Crimestoppers on 0800 555 111.
The injured man was taken to Harrogate District Hospital and then transferred to York District Hospital for treatment to his broken jaw.