The actor and writer, 56, is famous for his roles in comedy films such as Hot Fuzz, Shaun of the Dead and The World's End, and has also starred in TV series including Doctor Who and Spaced, and the Mission: Impossible movie series.
Pegg opened up about having the neurodevelopmental condition while appearing on podcast I'm ADHD! No You're Not on Thursday, saying he pursued a diagnosis as he "just wanted to know".
"There was a lot of build-up and suspicion, and it just took a while actually to get to get there," the actor said.
"It confirmed a lot of stuff that I'd been wrestling with, and that I'd been attributing to just character fault."
ADHD is described as a condition where the brain works differently, affecting a person's ability to concentrate, sit still, and control impulses.
Pegg has been outspoken about his mental health, including suffering from clinical depression and struggling to control his drinking, which he said he thought had "caused" his diagnosis.
"I felt like it was something that had come on in later life because I wasn't a naughty child," he said.
"I've had bouts of depression in my adult life since 18 and I thought maybe it had come on with that.
"Maybe over the years with therapy and medication - whatever - I've been treating the symptoms and not the cause of why I feel that way."
Pegg also explained how learning about his diagnosis put his childhood behaviour in perspective, and said: "I did feel different in a way I might not be able to put my finger on. Maybe everybody feels that way."
"I just assumed ADHD kids were kind of chaotic and difficult and didn't pay attention in school.
"But there were other things that I did do as a kid - fidget or whatever - that made me think, oh no, maybe I've had this longer than I thought."
11°C