The Beatles star said the experience - which came after he had taken drugs - had given him the "sense" that there was "something higher".
Speaking to The Sunday Times, he said: "I once took a drug, DMT... And I saw God, this amazing towering thing, and I was humbled."
Describing the encounter as a "clue", rather than a life-changing moment, he added: "It was huge. A massive wall that I couldn't see the top of, and I was at the bottom. And anybody else would say it's just the drug, the hallucination, but... we felt we had seen a higher thing."
Dimethyltryptamine - or DMT - is a class A hallucinogenic drug, which means users are likely to experience a distorted view of objects and reality and it can cause hallucinations.
The 76-year-old singer, who is due to tour his new music, also told the paper of the moment he believed his late wife Linda had "come back to give me a sign" after her death.
Describing seeing a white squirrel in the country, he said it was a "great moment", and added that he had allowed himself to believe that lost loved ones were "looking down" on him.
Sir Paul, who in July returned to Liverpool to perform in the Cavern Club for the first time since 1999, also spoke of teaching rapper Stormzy - who is a Christian, how to play the piano.
He told the paper: "He's looking to advance his music. As a rapper, I thought he'd have words down, but there was a piano, so I showed him basic stuff - how you get middle C, make a chord, a triad and, just by moving that, get D minor, E minor, F, G, A minor, and how that's enough for anyone."
Sir Paul's Freshen Up tour, which kicks off in Canada this month, will be the musician's first outing following the release of his new studio album Egypt Station, which is due to be unveiled on 7th September.
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