Fans at the SSE Arena were reportedly notified with an announcement at about 9pm, when thousands had already filled the arena waiting for the boys to come on.
The concert was cancelled because band member Liam Payne fell ill and could not go on stage.
Concert promoter Aiken PR said: "The show tonight has been cancelled as Liam has taken ill, and whilst not serious, he is unable to perform tonight."
Fans, who were understandably devastated, have been asked to hold on to their tickets and wait for further information.
A rep for One Direction said: "Unfortunately One Direction have had to cancel tonight's concert in Belfast. Liam has been taken suddenly ill, and the concert will sadly not be taking place.
"Fans have been asked to hold on to their tickets and there will be a further statement tomorrow. The band apologise to their fans and wish Liam well as soon as possible."
Deepest apologies to everyone at the show tonight in Belfast. Very disappointed, but unfortunately these things happen. We're very sorry. H— Harry Styles. (@Harry_Styles) October 20, 2015
Harry Styles, Niall Horan and Louis Tomlinson also took to Twitter to apologise to their fans, some of which had travelled from all over the world to see the chart-topping boy band perform.
Harry said: "Deepest apologies to everyone at the show tonight in Belfast. Very disappointed, but unfortunately these things happen. We're very sorry."
Niall tweeted: "So sorry we have had to cancel tonight guys. Unfortunately these things happen... Thank you to all who came down tonight!"
And Louis wrote: "We're so so sorry for what happened tonight, it was out of our control! Lots of love to Liam and thank you for being so understanding!"
One Direction's cancelled show was the first of three sold out concerts booked at the venue in Belfast.
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