The 23-year-old defender and Chelsea academy product helped the Royals win last season's npower Championship title and has made six appearances in the Barclays Premier League this season.
"I'm very excited to remain at a club that has such ambition in the Premier League," Cummings told the club's official website.
"To have already played against the likes of Man United and Liverpool is great experience at my age. I can't have asked for anything more.
"Everyone at the club wants to fight for their shirt and play in the Premier League and everyone knows what we've got to achieve to stay here."