Route 55 begins at the New York state line in rural northern Sherman, connecting with New York State Route 55 in the hamlet of Webatuck (ultimately leading to New York State Route 22).

It travels eastward for 1.6 miles (2.6 km) then intersects with the northern end of Route 39, which leads to the town center of Sherman.

Route 55 becomes Webatuck Road in New Milford, running for another 0.6 miles (0.97 km) until it ends in the Gaylordsville section of New Milford at an intersection with US 7 near the Housatonic River.

In the 1920s, the road between the New York state line (towards the hamlet of Webatuck) and the Gaylordsville section of New Milford was designated as a state highway known as Highway 129.

[2] The number 55 was chosen to match New York State Route 55 across the state line, which was also extended north from its original eastern terminus in Pawling to Wingdale (via an overlap with New York State Route 22) and then east through Webatuck to the Connecticut state line.