Officers were called at approximately 7.30pm on Monday, 13 January to reports of a man found collapsed suffering injuries on Windmill Road in Croydon.
The 77-year-old man was taken to a south London hospital for treatment. He remained in hospital but died from his injuries on Saturday, 1 February.
His next of kin have been informed.
A special post-mortem examination will be scheduled in due course.
Detective Sergeant Steve Andrews, of the Met’s South Area Command Unit, is investigating. He said: “We are currently treating the death of this man as unexplained and our thoughts are with his family at this very sad time.
“Despite thorough enquiries made so far, including speaking to witnesses and examining potential CCTV opportunities, we’ve not as yet been able to establish how the man came to sustain his injuries which included a broken arm and broken leg.
“We are keeping an open mind as to the circumstances of his death, including whether he may have been involved in a collision.
“Please make the call to us, or anonymously via Crimestoppers if you can help us determine what happened.”
Anyone who was in the Windmill Road area at around 19:30hrs on Monday, 13 January and can help provide information that could assist officers is asked to contact police on 101 and quote Cad 6441/13Jan.