The rapper unveiled the track, taken from her forthcoming Fantasea EP, via her Twitter and Tumblr accounts on Tuesday.
Listen to 'Neptune' - featuring fellow hip-hop artist Shystie - below (explicit content):
Fantasea will be released on July 11. It was originally scheduled for a July 4 release but was postponed for unknown reasons.
Banks recently announced that she has covered a track by The Prodigy which will feature on the free mixtape.
Watch Azealia Banks's video for 'Liquorice' below: