Exeter Chiefs' Sam Skinner also joins the squad. Fagerson and Skinner were both initially ruled out through injury.
However, impressive form for their respective clubs has propelled them back into Townsend's plans.
Gloucester second-row Alex Craig has been released from the squad back to his club.
There is still no place for Finn Russell, who was left out of Scotland's first two games after breaching team protocol.
Fagerson's last Test outing came the following weekend, in the away win over Georgia in Tbilisi, with the 21-year-old narrowly missing out on going to Japan.