State Route 100 (SR 100, also known as Robert Gray Drive) is a short state highway in Pacific County, in the U.S. state of Washington. It extends 4.38 miles (7.05 km) from U.S. Route 101 (US 101) in the city of Ilwaco, counterclockwise to end at itself in the same city, about 0.06 miles (0.10 km) from its clockwise end with US 101. The route serves as a connector to SR 100 Spur, which goes south from SR 100 to Cape Disappointment State Park at the southernmost point of the loop.

SR 100 was approved by the Washington State Legislature in 1991 along its current route. Even though SR 100 was approved by the Washington State Legislature and WSDOT in 1991, no signs went up until the law that made SR 100 took effect on April 1, 1992. The spur route was also established in 1991.

SR 100 runs 4.38 miles (7.05 km) from U.S. Route 101 (US 101) in the city of Ilwaco, counterclockwise to end at itself in the same city, about 0.06 miles (0.10 km) from its counterclockwise end with US 101. The route serves as a connector to SR 100 Spur, which goes north from Cape Disappointment State Park to SR 100 at the southernmost point of the loop. WSDOT has found that more than 2,200 motorists utilize the road daily after the intersection with US 101 based on average annual daily traffic (AADT) data.

SR 100 starts at an intersection with US 101 in Downtown Illwaco. It then becomes North Head Road and intersects 2nd Avenue SW, known as SR 100. After the intersection with 2nd Avenue SW, SR 100 goes west and becomes Robert Gray Drive. After turning south, SR 100 intersects SR 100 Spur, becomes Second Avenue SW and turns northeast towards Illwaco. Once in Illwaco, SR 100 turns north to intersect North Head Road, known as SR 100.

SR 100 became a state highway in 1991, and the highway designation took effect on April 1, 1992.

State Route 100 Spur (SR 100 Spur) is a 0.09 miles (0.14 km) long spur route of SR 100 that runs south from SR 100 at the southernmost point of the loop to Cape Disappointment State Park. The spur is signed north – south by WSDOT, unlike other highways in Washington, that are signed west – east or south – north. The highway is officially known as SR 100 Spur Canby, as it connects to Cape Disappointment State Park, formerly called Fort Canby State Park. More than 900 motorists use SR 100 daily based on AADT data.