The song doesn't use power chords but if you are playing it on guitar it sounds better just playing the root notes. Basically it's Verse and Bridge: A Bb A Bb A A A Bb A You can figure out the rhythm. The chorus is A Ab A Bb A Ab A repeated over again That's pretty much it for the chords. Lame song, I know, but catchy and chicks dig it which is why we all play guitar in the place.
Source: http://www.azchords.com/j/justintimberlake-tabs-2116/sexyback-tabs-356704.html