Internationally, it became Timberlake's first single to reach number one in the United Kingdom.
In Australia, the track was Timberlake's second number-one single, spending two consecutive weeks at the top.
Glenn Gamboa of Newsday was complimentary towards the song, writing that it was "Timberlake at his best, mainstreaming a combination of the wild, edgy dance-pop style of Basement Jaxx with Missy Elliott's more experimental hip-hop to create a boldly inventive sound that still pleases the masses." Kelefa Sanneh of The New York Times wrote: "...'Sexy Back', a twitchy, emaciated track that shot to the top of the charts and also became an irritating catchphrase.
(Suddenly everyone was bringing Adjective Back.)" Dagny Salas of North by Northwestern commented that Timberlake and Timbaland trade ridiculous lines, with "Get your sexy on/Go ahead, be gone with it" and "Dirty babe/You see these shackles/Baby I’m your slave/I’ll let you whip me if I misbehave." Salas continued in her review that it was not a song that should be good, "but does that really matter when it’s good in the way that songs are during the summer [...] That kind of good makes songs like 'Sexy Back' completely irresistible." Matt Burns of The Post wrote, "Take the polarizing summer single 'Sexy Back', a repetitive song with little structure that doesn't know when to stop, all centered on a ridiculous declaration.
Timberlake described "Sexy Back" as "an experiment gone right from the sort of synthesizer influence to the distorted vocals", adding that it was "one of the songs that the more you listen to it its just hook after hook after hook.
Its just one of those 'flow off the top of your head' tracks, in terms of melody.
The instrumentation used in the song includes a pounding bass beat, electronic chords, and drum machine sounds.
"Sexy Back" became Timberlake's first number-one single on the Billboard Hot 100, staying seven weeks at the top spot.
It also topped several of Billboard magazine's other charts, including the Mainstream Top 40, Hot Dance Airplay, and Hot Digital Songs, and entered the top ten on most singles charts.
And then there's the unique thing of Timbaland acting as a narrator, saying things like 'take it to the bridge', or 'yeah'. It's two guys interacting." Timberlake said that the chorus was "very James Brown-ish, call out and repeat, like 'Sex Machine'." He wrote the track "from top to bottom. In the Billboard magazine review of the album, contributor Katy Kroll wrote that Timberlake "claims to be bringing 'sexy' back to pop music, and indeed he is.
You can almost feel beads of sweat rolling off the title track and 'Sexy Back' featuring Timbaland".