The 21-year-old goalkeeper signed a new contract this summer but will spend the rest of this year with the League of Ireland Premier Division side until January 1st.
Norcott, who has made two appearances for the Royals, will be part of Derry City’s UEFA Europa League qualifying campaign starting Thursday (June 9) against CSKA Sofia.
A spokesman said: “We wish Tom and Derry City all the best for the remainder of the season.”
Norcott has previously been loaned to Woking, Farnborough and Enfield Town.