The 28-year-old, who has 21 international caps to his name, has put pen to paper on a three-and-a-half year deal at the Championship club.
Adrian Popa will wear the number 25 shirt for the Royals, and could be available tomorrow in Birmingham subject to international clearance. pic.twitter.com/fgNqe68p74— Reading FC (@ReadingFC) January 30, 2017
Playing in Jaap Stam's side will mark the first time that Popa has competed for a club outside of his homeland, having had spells at FC Universitatea Cluj and CS Concordia Chiajna.
After joining Steaua in 2012, the winger made more than 200 appearances, winning three league titles, and also has experience playing in the Champions League and Europa League.
"Adrian is a forward with enormous potential who has played at a high level in Romania as well as in European and international football," Stam told Reading's official website.
"He is an experienced player, who is quick, a constant threat to the opposition, good on the ball and who is comfortable playing on the left, on the right or even up front in a two.
"With the amount of games we have coming up, a player of his quality will make it easier for us to rotate a little more and give some players a rest if we need to, so I'm delighted to have him here as part of our squad."
The Royals currently sit third in the Championship, six points off the automatic Premier League promotion places.