Emergency services rushed to the scene near Streatham High Road at 3.50pm on Sunday.
They found the man suffering from a stab wound in Hopton Road.
An air ambulance was also scrambled as paramedics battled for almost an hour to save the victim.
He was pronounced dead at the scene at 4.39pm, Scotland Yard said. A man was arrested at the scene.
Video posted on social media shows a cordon in place and a number of police cars at the scene.
A spokesman for the Met Police said: “Officers are in the process of informing next of kin. Formal identification awaits.
“A post-mortem examination will be held in due course.
“A man was arrested at the scene on suspicion of causing grievous bodily harm. He has been taken to a south London police station where he remains in custody.