The 22-year-old son of former Newport director of rugby Lyn Jones started his career with the Dragons, playing in 25 games for the Pro12 club.
His last appearance came as a substitute in a 27-11 defeat by Treviso on 23 September.
"We are pleased to be able to bring in someone with Luc's ability," Quins director of rugby John Kingston said.
"Following an unfortunate injury to Calum Waters, it was important to secure the necessary depth at scrum-half ahead of a period of international unavailability."
Harlequins have signed former Newport Gwent Dragons scrum-half Luc Jones on a three-month deal.