The 33-year-old has signed a four-month contract to stay at the club until mid-January.
Carroll, who won nine England caps, scored two goals in eight appearances for Reading last season before joining West Brom in January.
Reading manager Paul Ince told the club website: "I am really pleased to have Andy with us.
"Of course, a striker of his prowess gives us power and a battling presence at the top of the pitch and I'm sure that will translate into goals.
"However, Andy's arrival also offers us another strong voice in the dressing room.
"He is a leader, both on and off the pitch, and having trained with us all this week I can already see the positive influence he will have on the players around him and the boost he will no doubt give to the fans."
Carroll made his name at hometown club Newcastle before joining Liverpool for £35million in January 2011.
He moved on to West Ham 18 months later and had a second spell at Newcastle before signing for Reading last November.