Easy On Me, a spare and emotional piano ballad, was released at midnight UK time, offering fans the first glimpse of her "divorce album", entitled 30.
Easy On Me sees Adele explaining her decision to walk away from her marriage in 2019, while asking her son and ex-husband for understanding.
That will be her follow-up to massively successful albums 19, 21 and 25.
Fans around the world stayed up to hear the song as soon as it premiered - with 299,000 people watching its debut live on YouTube.
It comes just five weeks before her new album - which was first teased in a global marketing campaign that saw the number 30 projected on to buildings and billboards in Brazil, Mexico, Dubai, Italy, Germany, Ireland, the US and the UK.
Fans correctly guessed the release date of 19 November when Taylor Swift moved her forthcoming album forward by a week, apparently to avoid a clash with Adele.
Like her previous three albums the title is a reference to a specific age in Adele's life.
Thirty is the age at which she married her long-term partner, Simon Konecki, and then left him.
The star has said the album was recorded to help her nine-year-old son understand their divorce the following year.
"I wanted to explain to him through this record, when he's in his 20s or 30s, who I am and why I voluntarily chose to dismantle his entire life in the pursuit of my own happiness," she told Vogue magazine.
"It made him really unhappy sometimes. And that's a real wound for me that I don't know if I'll ever be able to heal," she added.