A man believed to be in his 20s was bitten on his leg at North Steyne Beach in Manly about 6.20pm today.
A group pulled the man from the water and onto the sand and began first aid as they waited for emergency services to arrive.
Multiple emergency services, including paramedics, the intensive care ambulance, a helicopter and police, responded to the incident.
The man was treated for serious leg injuries and taken to Royal North Shore Hospital in a critical condition.
All 20 beaches on the Northern Beaches have been closed until further notice.
This will be reviewed on an ongoing basis.
It came hours after a suspected bull shark bit an 11-year-old boy's surfboard also on Sydney's Northern Beaches this morning, and another a 12-year-old boy was mauled at a beach in the city's east yesterday.
Authorities said a shark took an estimated 15-centimetre chunk out of the boy's surfboard at the netted Dee Why Beach, near Dee Why Point, about 11.45am today.
The boy, who was helped by lifesavers, managed to walk away from the close encounter unscathed.
The NSW Department of Primary Industries and Regional Development said photo analysis of the bite mark suggested it was likely from a bull shark.
Department crews, lifesavers and the local council were responding to the incident.
Jet skis were patrolling the waters as a drone monitored the area from above.
The incident came just a day after another suspected bull shark mauled a 12-year-old boy about 27 kilometres away near Shark Beach in Vaucluse, leaving him fighting for life at Sydney Children's Hospital in Randwick.
He suffered serious injuries to both legs and was pulled from the water by his friends and emergency services.
The three shark attacks come after the city was battered by a storm and heavy rain, which caused runoff.
Swimmers are advised to be careful, avoid the beaches, and stay clear of low-visibility and murky water.
Comment: Update January 24
9news.com.au reports: