The 33-cap loose-head, who was forced to retire from Scarlets duty last season, will work under forwards coach Rob Appleyard.
Thomas has been helping the Dragons on a consultancy basis throughout their pre-season preparations but he has now penned a one-year deal.
The Gwent region possess an abundance of emerging talent in the front row and it is hoped Thomas will be able to pass on his wealth of experience.
“Iestyn has a wealth of experience in the scrum and we are really looking forward to him working towards making our set piece not just competitive but dominant,” said director of rugby Robert Beale.
“He has just finished playing so has a lot of credibility with our players, many of whom have gone up against him.
“Iestyn is a WRU level three coach and while you could say he hasn’t got much experience, you only need to look at what he has achieved as a player to know that he will command respect.
“He would be the first to admit that he needs to polish up on parts of his delivery but he is working in a specific area that he knows a vast amount about.
“Steve Jones apart, we have a very young front row but there is a huge amount of potential there.
“He will be speaking as a prop to our props, hookers, second rows and back row forwards about what he expects and I am confident he will be a success.”
It has been reported that the Dragons were eyeing a move for the Pontypool man before he was forced to hang up his boots due to the persistent neck injury.
But he has cast his experienced eye over the Dragons pack on Beale is confident he will be able to build a solid set-piece at Rodney Parade.
“Iestyn has watched footage from last season and I think he will have a slightly different approach to Danny in how he wants the scrum to operate,” added Beale.
“There will be a bit of a bedding in, but I am confident that in time we will have a very good scrum.”
