The taxi was turning right on a red light at about 9:30 a.m., said Dustin Sternbeck, a spokesman for Arlington police.
The pedestrian a 76-year-old woman, sustained life-threatening injuries. She was crossing at Carlin Springs Road and North Glebe road, Sternbeck said.
The cab driver stayed on the scene and was being interviewed by police Wednesday morning.
This item has been updated.