The track, titled 'Day Four', is a relatively down-tempo song that has drawn comparisons on Twitter and Soundcloud with Silent Alarm's 'So Here We Are'.
Pitchfork reports that Australian station Triple J exclusively premiered the track, before the band tweeted a Soundcloud link to fans shortly afterwards.
The song will feature on upcoming album Four, to be released on August 21 on Frenchkiss.
The band have announced an intimate UK tour to accompany the record's release, with the following dates lined up:
October 12 - Newcastle, O2 Academy
October 13 - Leeds, O2 Academy
October 14 - Glasgow, O2 Academy
October 15 - Birmingham, O2 Academy
October 17 - Southampton, Guildhall
October 18 - Bristol, O2 Academy
October 19 - Manchester, WHP
October 20 - Cambridge, Corn Exchange
Listen to Bloc Party's 'Day Four' below: