The incident between a pedestrian and car happened in Brigstock Road at around 11.25pm on August 18.
Officers were called to the scene and found the pedestrian, a man, injured.
He was taken to hospital where he remains in critical condition.
The driver of the vehicle, a 37-year-old man, was arrested on suspicion of driving offences.
He has been bailed to a date in mid-November.
Officers are now looking for anyone who may have seen the incident to contact them on 020 8285 1574.
A spokesperson for Metropolitan Police said: “Police are appealing for witnesses following a serious collision in Croydon.
“Officers were called to Brigstock Road in Thornton Heath at about 11.25pm on Friday, 18 August.
“It was reported that a car had collided with a pedestrian.
“The pedestrian, a man aged in his 60s, was taken to hospital where he remains in a critical condition.
“The driver of the vehicle, a 37-year-old man, was arrested on suspicion of driving offences.
“He has been bailed to a date in mid-November.
“The investigation is being led by officers from the Met’s Serious Collision Investigation Unit.
“Any witnesses or anyone with any information is asked to call police on 020 8285 1574 or contact via Twitter @MetCC.
“Please quote CAD 8850/18Aug.
“In particular, officers are keen to hear from anyone who has dash cam footage of the collision or the events leading up to it.
“To give information anonymously contact Crimestoppers on 0800 555 111.”
