- Southern Terminus:
- North Avenue (US 1) and Cross Street (US 1) in Norwalk
- Northern Terminus:
- New York-Connecticut state line
- 8.37 miles
CT 123 is a north/south route through Norwalk and New Canaan. It begins at Cross Street (US 1) in Norwalk and runs north along Main Street for about half a mile. It then turns west onto New Canaan Avenue, passing under the US 7 expressway and then curving to the northwest and heading out of Norwalk. After about a mile and a half, it crosses the Merritt Parkway (CT 15) and changes names to become New Norwalk Road. It continues northwest to New Canaan. North of New Canaan, it curves to the north and changes names to become Smith Ridge Road, which extends north to the New York-Connecticut state line and continues into New York as NY 123.