The Beatles' "final" song, "Now and Then," has finally been released, decades after the death of its writer, John Lennon.

Featuring the sound of all four members of the British rock group - Lennon, Paul McCartney, George Harrison and Ringo Starr, the track reached No. 1 on U.S. iTunes shortly after it dropped Thursday, according to @chartdata.

In a short film on the making of the song released on YouTube Wednesday, the band revealed that the song was originally written and recorded by Lennon at his New York City home in the late 1970s.

After he died in 1980 at the age of 40, Lennon's wife, Yoko Ono, gave the demo to the remaining Beatles members in 1994.

McCartney produced "Now and Then" with Giles Martin, and the final version features Lennon's original 1977 piano and vocal, Harrison's guitar from the 1995 "Anthology" sessions, and Starr's singing and drumming in 2022, Rolling Stone reported.

"I know it's true / It's all because of you / And if I make it through / It's all because of you," Lennon sings.

"Now and then I miss you / Oh, now and then I want you to be there for me / Always to return to me," he and McCartney sing in the chorus.

Martin told Rolling Stone in an interview that while Lennon did not write the song for McCartney, the final version sounds like it was "in a very emotional way."

The English record producer and songwriter went on to describe "Now and Then" as "bittersweet" and "very John."

Harrison, who died in 2001, said in the newly released short film: "To hear John's voice, that's a thing that we should cherish, and I'm sure he would've really enjoyed that opportunity to be with us again."

"It was the closest we'll ever come to having him back in the room," Starr added.

Lennon's son Sean also opened up on how he felt about his father's former bandmates working on the song following his death.

"It was incredibly touching to hear them working together after all the years that my dad had been gone," Sean said.

"It's the last song that my dad and Paul, and George, and Ringo, will get to make together," he added. 

"Now and Then" received praise from artists and fans alike.

Responding to an Instagram post by The Beatles sharing the song's release, singer Tony Harnell wrote: "LOVE the song! LOVE the production. What a beautiful day it is when new Beatles music is released!"

"So unbelievably beautiful and emotional," vocalist Mollie Marriott commented.

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The physical release of "Now and Then" drops Friday, in vinyl on 7-inch and 12-inch, with both black and colored vinyl. Fans can also purchase a limited-edition cassette single that includes "Love Me Do," the band's first single from 1962.

"Now and Then" will complete the new expanded version of the legendary "Red" and "Blue" albums, coming out on Nov. 10.