Thomas, 23, played for Wales Under-20s for two seasons and returned to Wales from Gloucester by signing for the Scarlets last autumn.
The front rower has played eight games for the region since his switch to Parc y Scarlets, before going out on loan to Bristol at the start of the current campaign and will provide much needed cover for the injured Brok Harris.
“I would like to thank the Scarlets and Wayne Pivac for allowing Nicky to come to us,” said Dragons head coach Bernard Jackman.
“Nicky had been at Bristol over the past few months and he made a big impression on and off the pitch there. He played with a lot of our players at age grade level and is one of the best scrummaging tight heads around.
“Our scrum has been a real strong point for us this season and we want to continue to dominate in this area and I feel Nicky has the attributes to do that. He isn’t available for Friday night against Newcastle, but we will be able to call on him for the two big back-to-back Welsh derbies at Rodney Parade over Christmas.”
Born in Swansea, Thomas also played five times for the Ospreys before graduating through the Hartpury College academy to join Gloucester.
Harris joins the lengthy Dragons injury list after picking up an injury in the Guinness PRO14 clash with Ulster at the start of this month, while Ollie Griffiths has also been ruled out for eight weeks as he faces ankle surgery after leaving the field against Russian outfit Enisei last Friday.
