The former Hull City player sustained a fractured skull in a nasty clash of heads while playing against Chelsea on 22 January, 2017.
Now medical specialists have advised Ryan a return to professional football would be too much of a risk.
Chief Executive of Headway, Peter McCabe said: "We were sad to hear the news that Ryan has been forced into retirement following his serious head injury.
"At Headway we understand the long-term impact a brain injury can have and although this difficult decision must be heartbreaking for Ryan, it is a sensible one.
"We applaud Ryan for following the advice of medical experts, and setting a good example to other players who sustain head injuries on the pitch.
"We will continue to offer support to Ryan and his family going forward."
In a statement on Hull City's website, Ryan said: "I have worked tirelessly in order to be able to return to the pitch. Unfortunately, having taken expert medical advice I now have no option but to retire due to the risks involved given the nature of my injury.
He added: "I’m retiring from the game with my head held high having done everything in my power over these past 13 months to have given myself every opportunity to return to the pitch.
"I feel content knowing that I dedicated my life to be able to play football at the highest level, hard work really does pay off."
Since sustaining his head injury, Ryan has done a great deal of work to support Headway.
When Ryan's former teammates at Tottenham Hotspur auctioned off signed t-shirts, Ryan requested that the money go to Headway.
In November 2017, he presented Headway with a cheque for more than £500 raised by Hull City Official Supporters Club members, who put on a series of quiz nights and a raffle.
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