The pedestrian was hit in Dandenong about 2.15pm yesterday.
Police arrested a 33-year-old male in Hallam about 11pm last night.
Detectives seized a car and trailer during the arrest.
He remains in custody and is assisting police with their enquiries.
Investigators cordoned off the area of the collision at Clow Street between Thomas Street and Robinson Road.
The 24-year-old victim remains in hospital.