The 20-year-old, Town's current young player of the year, has made 25 appearances this season and scored five goals.
Director of Football Nick Hammond said: "This is a very good deal for us, Sean is a young centre-half with decent league experience for his age, but there is also lots of improvement to come from him and he is one for the future."
The Royals have also snapped up 23-year-old striker Brett Williams from non-league Eastleigh.
Hammond added: "This is a very interesting deal, and reminiscent of when we signed Michail Antonio, who has gone on to show great development over the past two years.
"Brett is a touch older, but we think he has got something and being in a full time environment gives him the opportunity to develop."