Thomas, who won 33 caps for his country, had been working the Welsh region on a consultancy basis throughout their pre-season preparations but has now penned a one-year deal.
And Dragons director of rugby Robert Beale is already looking forward to seeing the imrpvoements the scrum can make under Thomas's tutelage.
"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," he said.
"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."
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