The Biebs opened up about life as a married man Tuesday night at London's O2 Arena where he held a Q&A sesh. It's kind of funny ... as he's riffing about the challenges that come with marriage, you hear someone in the audience say, "I don't see no ring."

Either way ... Justin says life with Hailey's awesome since tying the knot in 2018. They love 'em some Netflix, but he confessed they also get easily, ahem ... distracted. Marriage itself, he says, is no easy task and gives the young crowd some marriage advice.

Justin also says his new album, "Changes" -- set to drop Friday -- reflects some of the hardships and changes in his life. Marriage, being a huge change, and at least one of the hardships has to be health issues.

As we first reported ... Justin contracted Lyme disease, which went undiagnosed for most of 2019. He recently opened up about it on the latest episode of his YouTube docuseries.