The emergency services were called to a shop, believed to be Edi Food and Wine, on Evering Road shortly before 5pm on Tuesday (April 29).
The victim was taken to hospital with stab wounds that have been deemed not to be life changing.
A 25-year-old male suspect has been arrested on suspicion of grievous bodily harm and is in police custody.
An air ambulance landed on nearby Hackney Downs Park following the incident while people were out enjoying the sunshine.
Detectives are appealing to the public for information that could help their enquiries.
A Metropolitan Police spokesperson said: “Police and the London Ambulance Service were called to a shop on Evering Road, Hackney, on Tuesday, 29 April at 16:53hrs following reports of a stabbing.
“A 40-year-old man was taken to hospital for stab wounds, his injuries were deemed non-life changing.
“A 25-year-old man was arrested on suspicion of grievous bodily harm and remains in police custody.
“Anyone with information can call police on 101 quoting CAD 5515/29APR.
“Alternatively contact Crimestoppers anonymously on 0800 555 111 or visit crimestoppers-uk.org.”
A London Ambulance Service spokesperson said: “We were called on Tuesday 29 April at 4.50pm to reports of a stabbing in Evering Road, Hackney, N16.
“We sent resources to the scene including ambulance crews, a medic in a response car, an incident response officer and London’s air ambulance.
“We treated a man at the scene before taking him to a London major trauma centre.”
