The 30-year-old winger spent the second half of the 2021-22 season on loan with the Royals and scored twice in 15 appearances.
He arrived a month prior to the appointment of his father Paul as Royals manager.
"His talent has always spoken for itself," boss Paul told the club's website.
"Thomas is my son. But, importantly, he is a Reading player the moment he arrives at the training ground every morning.
"He still wants to improve, he still wants to succeed and he proved his worth here at Reading in the latter stages of last season."