The Met Police said it was called at about 02.25am. The man was taken to hospital by the London Ambulance Service but died a short time later.
The Met say a crime scene remained in place and no arrests had been made.
A Metropolitan Police spokesman said: “Enquiries are ongoing to inform next of kin.
“At this early stage it is believed the man had suffered a gunshot injury.”
Passersby in and around the Croydon Road and Beddington Lane area on Monday night who saw anything suspicious are urged to get in contact with police.
Motorists with dash cams are asked to check their footage to see if anything relevant to the inquiry has been captured.
Anyone with information is asked to call 101, quoting CAD616/4Apr, or independent charity Crimestoppers anonymously on 0800 555 111.
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