The pedestrian was struck by a lorry at the Moira Road roundabout on the Glenavy Road at 07:30 GMT on Wednesday.
Police have appealed for anyone who witnessed the incident to contact them.
The A26 Glenavy Road will be closed from 19:00 GMT to 20:00 GMT on Wednesday for a reconstruction and delays are expected.
A man has been killed in a collision in County Down.