The 'Beauty and a Beat' singer made an unsettling fashion choice to go shopping near Piccadilly Circus during some downtime from the UK leg of his 'Believe' world tour.
The 18-year-old also posted a black-and-white photo of himself wearing the gas mask on Instagram, which came with the caption "Mi eht tseb", an anagram of 'I'm the best'.
However, Bieber made no attempt to explain why he donned the unorthodox headwear.
The star kicked off his tour at the Manchester Arena on Thursday (February 21) and played in Liverpool's Echo Arena on Sunday (February 24).
His next UK concert is at the National Indoor Arena in Birmingham tomorrow (February 27). He will return to London for a four-date stint at the O2 Arena on March 4.
It has been rumoured that Bieber is set to slip on another piece of headgear - an astronaut's helmet - after being offered a space trip by NASA.