The Liverpool centre back says he is calling time on his career now to avoid speculation in the months ahead.
He told the club's website: "I'm making this announcement now because I don't want the manager or the club to be answering questions on my future when I've already decided what I am going to do.
"I will be fully committed between now and the end of the season to doing the very best for Liverpool, as I have done my entire career since joining aged just nine years old.
"It has been a privilege and an honour to represent this great club for as long as I have and I am immensely proud to have done so and thankful for all the support I have had. There are many memories I want to share and people to thank, but now is not the time for that."