Soldiers from 3rd Battalion The Royal Welsh, whose regular jobs include plumbing and carpentry, are taking on Tower Guard duties for a week.
They include a daily march to the Byward Tower and the performing of the Ceremony of the Keys.
The tradition has taken place for 700 years.
Reservists from Bangor, Powys, Swansea and Newport are taking part.
Army reservists have swapped their day jobs for guarding the Crown Jewels for the first time in the Tower of London's history.