The 28-year-old said in an Instagram video that the condition is due to a diagnosis of Ramsay Hunt syndrome.
"As you can see this eye is not blinking. I can't smile on this side of my face ... so there's full paralysis on this side of my face," he said.
Ramsay Hunt syndrome is when a shingles outbreak affects the facial nerve near someone's ears, say medical experts.
Earlier this week, Bieber's Justice World Tour - which began in February - announced three shows would be postponed.