The incident happened outside Squire's carpet shop on Victoria Road just before 7am on Friday April 3.
A man who was barechested and wearing tracksuit bottoms was reported to have violently confronted the victim and tried to throw a bag of vomit over him.
The off-duty officer and another man managed to detain the suspect until back-up arrived.
He was arrested on suspicion of assaulting an emergency worker.
Witnesses or anyone with information that could assist the investigation are asked to call North Yorkshire Police on 101, select option 1, and speak to the Force Control Room.
Please quote reference number 12200054533 when providing details about this incident.