Route 166 is a minor state highway in southern Connecticut running from Westbrook to Old Saybrook.

It heads east, roughly paralleling I-95 on the north, into the town of Old Saybrook.

In Old Saybrook, it turns south, crossing I-95 along the Rosario Aloisio Memorial Bridge, with an interchange at exit 66.

It continues southeast over the Amtrak Northeast Corridor line to end at an intersection with US 1 in the western part of Old Saybrook.

[1] Route 166 was established from town roads in 1935, and has had no major changes in alignment or designation since.