It brings to an end Roberts' five-and-a-half-year stint with Blackburn where he made over 150 appearances, scoring 29 goals. The 34-year-old had fallen out of favour under Rovers boss Steve Kean due to a contract dispute, with the last of his 12 appearances this season just under seven weeks ago.
A statement on Rovers' official website www.rovers.co.uk: "Jason Roberts has signed for Reading Football Club. The 34-year-old centre forward links up with the Championship club, with the terms of the transfer undisclosed."
Despite a troubled last few weeks, with Roberts declaring his frustration at his lack of football, he leaves Ewood Park on good terms.
Via his Twitter account, Roberts said: "I have enjoyed every moment of my time at the club. I have so many friends at the club that I will always want the best for BRFC. I wish the fans, staff and players success now and forever!"
Royals chairman Sir John Madejski believes manager Brian McDermott has pulled off a transfer coup.
Speaking on the club's website, Madejski said: "I am delighted a man of Jason Roberts' calibre has decided to sign for Reading. He will bring enormous experience to the club, and I wish him every success in helping his new team-mates."
Roberts' capture is part of a double header for Reading, who have also confirmed the signing of Derby midfielder Tomasz Cywka.
The 23-year-old has joined on a free transfer until the end of the season, turning down a more permanent move to his native Poland with Slask Wroclaw.
Madejski added: "I'd like to welcome Tomasz to Reading. This move gives him the opportunity to show everyone his talents."