- Western Terminus:
- Hempstead Turnpike (NY 24) in Farmingdale
- Eastern Terminus:
- Montauk Highway (NY 27A) in Babylon
- 7.08 miles
- Nassau, Suffolk
NY 109 begins at Hempstead Turnpike (NY 24) in Farmingdale as Fulton Street. It runs to the southeast. Upon crossing the Nassau-Suffolk county line, it changes names to become Babylon-Farmingdale Road. There are grade-separated interchanges with NY 110, the Southern Parkway, and Sunrise Highway (NY 27). In Babylon, the divided highway ends, and NY 109 merges into Little East Neck Road, which is a local street to the north. It continues south to Montauk Highway (NY 27A), where it ends.