State Highway 78 or SH 78 is a state highway that follows surface roads in a predominantly southwest-to-northeast direction in the Dallas before traveling 90 miles (145 km) north-northeast to the Oklahoma State border.

The highway's southern terminus is at Interstate 30 in Dallas. From there, it follows Grand Avenue along White Rock Lake and then Garland Road into Garland as it passes Interstate 635. In downtown Garland, Highway 78 follows the central streets of Avenues B and D before joining Lavon Drive and moving northeastward, where it formerly was the eastern terminus of the President George Bush Turnpike, which has since been extended to Interstate 30.

It continues to the northeast through Sachse and Wylie, crossing between Lake Ray Hubbard and Lake Lavon. It passes through Lavon before turning north through Farmersville and Blue Ridge. At SH 160, just before SH 121, SH 78 turns east, continuing to Leonard before turning northeast again to Bailey. At Bailey, SH 78 turns north and cuts through Bonham (as Center Street) just before it reaches the border with Oklahoma, where it crosses a 1938 truss bridge over the Red River. The route continues as Oklahoma State Highway 78 through Durant to its terminus at Tishomingo.

The highway was originally designated in 1926 along its current route. Part of this route was previously designated as SH 5C.

SH 78A was a spur designated by 1928 from Desert to Denison. This spur was renumbered as SH 160 by 1933.

The section of the highway between I-30 and SH 66 used to be part of U.S. 67 until sometime in the 60s.

In 1990 SH 78 was flooded, as engineers at the Lake Lavon Dam decided to release the water from the lake into the East Fork of the Trinity River. For a week and a half, the traffic going between Wylie and the Eastern portion of Collin County was diverted to go through the city of Rockwall, a nearly 20-mile (32 km) detour. The railroad that ran adjacent to the highway was completely washed out, and significant shoulder damage was done to the highway.