The Wales lock, who impressed throughout the RBS 6 Nations, takes the captain's armband for the first time following Lewis Evans' injury.
Coombs skippered the Dragons during pre-season but the Millennium Stadium clash will be the first time he has taken on the leadership duties in a competitive regional match.
"It's been something that I've wanted to do for such a long time, since coming into the Dragons set-up after captaining Newport," said Coombs.
"You are always looking for the next box to tick, I managed to do a nice one a few weeks back with Wales and it will be the same on Saturday.
"It's going to be a proud moment and it will be another very important occasion for me.
"I always have a lot to say in the changing rooms before a game anyway, so my role doesn't change there.
"But I don't think I will need to say much on Saturday, it's a massive occasion and a great opportunity for the boys. There's real excitement building already."
Evans tore his calf in the 52-19 defeat at the hands of the Ospreys last Friday and remains a doubt for the rest of the season.
But Dragons head coach Darren Edwards reckons Coombs is the perfect replacement.
"As we saw when he put on that Wales shirt, he leads by example," said Edwards.
"Coombsy demonstrates what he expects of others through his actions and has a huge amount of respect from the rest of the players and the entire coaching staff.
"I am really proud to be sending him out as our captain and I don't doubt he will lead the team with everything he has got.
"There will be no backward step from Coombsy and he will demand that the rest of the team stand shoulder to shoulder with him."