He said he had been undergoing assessment in recent months and had now been officially diagnosed with the condition.
The NHS says ADHD "is a condition that affects people's behaviour" and that people with the condition "can seem restless, may have trouble concentrating and may act on impulse".
Telling fans it wasn't "an easy one" to write, the 31-year-old said: "I have suspected for most of my teenage and adult life that I’m not the same as a lot of the others around me."
He explained: "I struggled when others did not, and thrived where others found challenge.
"I have tried to hide and also conform for most of my life. Why? My fear has always been that I don’t want to be labelled and judged. Why would I want to be known as having a 'disorder'?"
The TV star – who rose to fame in the 2018 series of Love Island – said after a talk with musician Toddla T he decided to "stop burying my head in the sand" and look for answers.
George said he wasn't a fan of the word "disorder" and that it had been a "weird few weeks" coming to terms with his diagnosis.
"I'm not exactly sure why, I'm the same Alex and I shouldn't feel any different," he said.
"I think some of the reflection and even sadness comes from a realisation that I'm 31, and only now beginning to understand why I am as I am.
"How many other young Alex's are out there, who are asking themselves 'What's wrong with me? Why am I different?' or even worse being punished for being them."
The star said he still needs time to "reflect and gather my thoughts".
"But I really feel uncomfortable with the naming of this diagnosis," he said.
"Am I disordered, the 'problem', or is it that I just don't fit neatly into the box society expects me to? That's the question I'd like to understand the answer to."
Watch: Dr Alex George talks about his unlikely appearance on OnlyFans