The road is closed between Hayter Road and Water Lane following the incident at about 9.45am.
Buses are on diversion, but severe delays are expected in both directions.
The Twitter feed @LeanderRdSW2 wrote: “woman was hit and very badly hurt by car when she crossed road after getting off bus opposite Lambeth College.”
Another user, Melissa Horton, said: “Nasty accident on Brixton Hill near junction with Brixton Water Lane. Road being taped off and traffic diverted.”
An update from Transport for London at 10.20am said: “Traffic on Brixton Hill northbound is currently queuing from Dumbarton Road and southbound traffic is queuing from St Mathew’s Road. Expect delays.”