ABBA won the 1974 Eurovision Song Contest with Waterloo and had a sequin-spangled string of hits including Dancing Queen and Take a Chance on Me before splitting up in 1982.
The band's statement said the members reunited to plan a virtual tour featuring digital avatars, and decided to go back into the studio.
ABBA said "it was like time had stood still and that we had only been away on a short holiday. An extremely joyous experience!"
Swedish pop group ABBA celebrate after winning the 1974 Eurovision Song Contest on stage at the Brighton Dome in England with their song Waterloo. Photo / AP
I Still Have Faith In You is due to be performed by the group's holograms in a December TV special broadcast by the BBC and NBC. There was no word on when the second track will be released.
Ulvaeus revealed earlier this month that digitally created virtual band members — "Abbatars" — would perform in a television show in 2018, followed by a tour in 2019 or 2020.
The band members have performed together just once since the 1980s, at a private party in 2016, and have long said they will never tour live together again.