The huge posters, which were seen in London, Tokyo, New York, Los Angeles and Australia, bore the words "Do You Know Who You Are", with the only clue they were promoting the 25-year-old star being his signature motto, TPWK - Treat People With Kindness - also spotted on the bottom.
The psychedelic video, which was filmed in Mexico, shows the topless singer dancing with a group of people, riding on the back of a motorbike, and mysteriously floating in water.
Many posted messages on social media expressing their delight - and some said they were so overwhelmed that they were actually in tears.
"Harry Styles, the only one to make me cry over and over of excitement when ever he releases a new song", said another.
Simultaneously, a preview for the music video also appeared on his official Vevo account at 11:30am AWST with a countdown clock set for 12:01pm.
I tweeted, 'Can't believe I have started dating again when Harry Styles exists - it's all futile, like what's the point, I will never fancy anyone even 1/100 as much.' And, to be honest, I really don't know how I'll be able to concentrate for the rest of the day. Harry croons in the song, which is backed by a soulful choir that prods you to do some soul-reflection yourself.
Styles unveiled his self-titled debut album in 2017, and it went to the top of the charts in the United Kingdom and the US. Styles is no stranger to statement lead singles: He introduced his debut albumHarry Styleswith "Sign of the Times", a almost six-minute-long piece of feathery, melodramatic rock balladry.