"Obviously this has been a very difficult decision," said the former Bury, Oldham and Sheffield United man.
"I absolutely love football but this is the right thing for me professionally, medically and also personally."
Armstrong added: "With the illness, both the medical team and I didn't feel I could regularly perform to the level you have to in the professional game.
"I've been very fortunate to play for Reading and all my other clubs, I've been lucky to have a career that I know many people would be delighted to experience. I'll miss being a part of the game on a daily basis."
Reading manager Brian McDermott said: "Chris has been an inspiration to me.
"Over a long period I've had many conversations with him. He's a quiet, unassuming man who sums up everything that is good about human beings. Nobody could have tried harder and given more than he has given."
Source: BBC Sport
Source: BBC Sport