- Western Terminus:
- Northern Parkway and Veterans Highway (NY 454) in Hauppauge
- Eastern Terminus:
- NY 25A in Mount Sinai
- 14.48 miles
NY 347 begins at the eastern terminus of the Northern Parkway in Hauppauge. At this point, it is multiplexed with Veterans Highway (NY 454). A short distance east of the Northern Parkway, NY 347 splits off from Veterans Highway (NY 454) as Smithtown Bypass and runs to the northeast. When it crosses the Smithtown-Brookhaven town line, it becomes Nesconset Highway. It ends at NY 25A, just east of NY 112 in Port Jefferson.