| State: | FL |
| Type: | FL |
| Route: | 666 |
| Map Custom: | yes |
| Alternate Name: | Tom Stuart Causeway / 150th Avenue |
| Length Mi: | 1.391 |
| Length Round: | 3 |
| Direction A: | West |
| Terminus A: | in Madeira Beach |
| Direction B: | East |
| Terminus B: | in Seminole |
| Counties: | Pinellas |
| Previous Type: | SR |
| Previous Route: | 659 |
| Next Type: | SR |
| Next Route: | 674 |
State Road 666 (SR 666) is a short state road in Pinellas County. Locally known as Tom Stuart Causeway, Welch Causeway, or Madeira Beach Causeway, the route crosses Boca Ciega Bay from Seminole to Madeira Beach. The bridge crossing Boca Ciega Bay is a bascule bridge with 11 spans, built in 1962. The route ends with an interchange with U.S. Route 19 Alternate (US 19 Alt.) in Bay Pines (originally built for the formerly proposed Pinellas Belt Expressway). The southwestern terminus of the route is at State Road 699 in Madeira Beach, in front of the Ocean Sands hotel.