"Skyfall" is an orchestral pop and pop-soul song with a duration of four minutes and 46 seconds. Epworth stated that although "Skyfall" is an original composition, he and Adele had worked to capture "the James Bond feeling" of previous theme songs. The song intentionally references Monty Norman's "James Bond Theme" after the first chorus. Norman said that the song had his seal of approval and that including the leitmotif he wrote for the Bond character was "a pretty sensible thing to do, if you want to feel the 'James Bond quality' of the music". Epworth said that while "Skyfall" was his first experience writing film music, he had been involved with the production of James Bond music before: while Epworth was a tape operator at AIR Studios, he recorded some film soundtracks including David Arnold's Tomorrow Never Dies.

Epworth stated that the producers' request was for "a dramatic ballad", so he and Adele tried to "do something that was simultaneously dark and final, like a funeral, and to try and turn it into something that was not final. A sense of death and rebirth". Epworth watched the first thirteen Bond films seeking the "musical code" of the songs, "whatever the modal structure or the chord that always seemed to unify those songs" and contributed to the mood and "that kind of '60s jazzy quality". Epworth identified as a uniting factor "a minor ninth as the harmonic code ... the Bond songs, they have that elaboration to it" and wrote what would become the instrumental part of "Skyfall". He described it as "a bit of a 'Eureka!' moment".

The chordal structure of the chorus of "Skyfall" was originally composed by American artist Matthew Shelly, as part of a prog-rock composition entitled "Failing Grace", a track from the album "Pain", released in 1991. "Skyfall" was composed in the key of C minor using common time at 75 beats per minute (Adagietto). Adele's vocal range spans over one octave, from the low note of G3 to the high note of C5, on the song. Heavily pregnant at the time of recording, Adele has commented that this was the reason for the song's low range, and has often struggled to perform the song live due to the lower register. The lyrics closely follow the plot of the film rather than focusing on romanticism. According to Epworth, the song is about "death and rebirth", saying "It's like, when the world ends and everything comes down around your ears, if you've got each other's back, you can conquer anything. From death to triumph, that was definitely something we set out to try and capture". The Daily Telegraph writer Neil McCormick described the lyrics as "slightly sinister" and containing references to a number of Bond tropes and motifs.

The phrase "Skyfall" in this Adele's song is sung with the same melodic contour and rhythm as "Blue Skies" in Irving Berlin's 1926 classic, making the two eponymous phrases strikingly similar in their musical delivery.

The theme song, and the identity of its singer, was kept secret, but rumours of Adele's involvement still emerged. Adele first mentioned recording a "special project" in September 2011 in an interview on The Jonathan Ross Show, leading to speculation in the media that she was recording a Bond theme. In an interview with NRJ in April 2012, the singer stated that she intended to release a new single by the end of the year; however, it would not be preceding a new album. The song's title was originally rumoured to be "Let the Sky Fall". In September 2012, OneRepublic vocalist Ryan Tedder posted a message on Twitter claiming he had heard the title track and that it was "the best James Bond theme in his lifetime". Adele's publicist, Paul Moss, mentioned the song on his Twitter feed; both later deleted their messages. The artwork for the song was leaked online, but Adele's involvement in the project was not officially confirmed until 1 October. "Skyfall" was released at 0:07 BST on 5 October 2012 as part of the "Global James Bond Day", celebrating the 50th anniversary of the release of Dr. No, the first James Bond film. A 12-inch single featuring unofficial remixes of the song was sold in Germany to promote the song.

The song was not included on the soundtrack album, marking the second time in the Bond series that the theme song was split from the soundtrack album, following "You Know My Name" from Casino Royale in 2006. Wilson and Broccoli still asked composer Thomas Newman to include a reference to "Skyfall" in the film's score "so that it didn't appear as a kind of 'one off' at the top of the movie". Newman opted to include an interpolation in the track "Komodo Dragon", used in a scene where Bond enters a casino in Macau. According to Newman, the scene had "a real moment of 'Bond' swagger", and the music fit the scene accordingly. Epworth was visited by Newman for advice, and Redford, who was already doing the score's orchestration, was requested to arrange "Komodo Dragon"; Newman was unable to do the arrangement because he felt that his task "was already so huge and daunting".

"Skyfall" received widespread critical acclaim. Entertainment Weekly wrote that there is "finally" a great James Bond theme. The Huffington Post described the song as a "brassy and soulful tune [that] fits perfectly alongside the work of Shirley Bassey in the oeuvre of James Bond title tracks". RedEye gave the song four out of four stars and declared that it "is a return to form, and if it doesn't get you hyped for the movie, you're not a Bond fan". The Daily Record named "Skyfall" its "Single of the Week" and gave the song five stars out of five. PopCrush gave the song four-and-a-half stars out of five and called it "wholly satisfying and worth the wait". Idolator wrote that "during the song's final third, Adele does, in fact, make the sky fall, in typical Adele fashion". Consequence of Sound commented that "rousing instrumentation elevates the vocals to soaring heights", while HitFix called the song a "majestic ballad" and a "classic James Bond theme". Newsday was also very positive, writing that "Skyfall" is "unlike anything else she's done in her young career. It's self-assured and grand, drawing inspiration from Dame Shirley Bassey, while adding her own powerful phrasing to make it her own. Adele's style so far has been to downplay her massive voice with lyrics that are questioning and self-deprecating. On 'Skyfall,' though, it sounds like the diva point of view suits her, too".

The Los Angeles Times complimented the song and said that the song "tells good things for this winter's blockbuster-to-be. It's not a reimagining or a musical departure, but simply a righting of the ship. The song is big, bold and seems to have a little spot-o-fun". MTV was also positive, stating that "Adele's lush song fits right in with classics by Shirley Bassey, Paul McCartney and Carly Simon". The Wall Street Journal felt similarly, writing that the song "has sweep and drama, [with] orchestral support [that] gives it a classical timelessness that sets it apart from typical pop songs. Because it is a theme for a Bond film, after all, the song is also shot through with the threat of violence and death". E! Online wrote that Adele's song was "a cross, and a good one at that, between the 1971 Bassey classic and a more-focused version of Garbage's 'The World Is Not Enough'." The Hollywood Reporter wrote that the song "instantly feels like a Bond theme, with the singer's sultry voice set against a minor chord progression. Done in big, orchestral style, the mood – like the singer – is all 1960s throwback, back when Bond themes like 'Goldfinger' were smooth, seductive and larger than life".

Jim Farber of the New York-based Daily News wrote in his review: "It suffers from a similarly meandering melody and ponderous progression. The grandeur of its arrangement easily upstages the tune", but "even so, the luster of Adele's tone, and the bravura arc of her vocal, makes it enjoyable enough. And, fifty years down the line, isn't that all we really expect from a Bond product these days?" Yahoo!'s Rob O'Connor gave the standalone song a positive review but felt that it was too soon to tell how the song would fit into the wider canon of Bond theme songs. Neil McCormick of the Daily Telegraph was less complimentary, describing the song as "classy" but at the same time, "overly predictable".