The 2012 Chemistry and Sports Science graduate will replace the injured Ali Price as Scotland bid to advance from Pool A which also features hosts Japan, Ireland, Russia, and Samoa.
Following his selection, Edinburgh scrum-half Pyrgos will be looking to make a second World Cup appearance after his call up in 2015 and will hope to add to his 27 Scotland caps to date.
Scotland Head Coach, Gregor Townsend, said:
“We’re disappointed for Ali [Price] to have to return home so early in the tournament.
"Both Ali and Hamish (Watson) had invested a lot of effort into being in their best physical shape for the world cup and it’s a shame they’ve only been involved in one game.
"However, we have a lot of belief in our wider group and the two new players [Magnus Bradbury and Henry Pyrgos] who have been given this opportunity.”