Interstate 635 (I-635[a]) is a 37-mile-long (60 km) partial loop around Dallas, Texas, in the United States between I-20 in Balch Springs and SH 121 at the north entrance of the DFW Airport in Grapevine.

The roadway is named after Lyndon B. Johnson, the former U.S. senator from Texas and the 37th vice-president and 36th president of the U.S. Where I-635 ends at I-20, I-20 continues the LBJ Freeway designation heading west.

Six miles (9.7 km) later, it takes a general westward turn as it intersects US 75 at exits 19A and 19B at the High Five Interchange.

The section from I-35E to US 75 (Central Expressway) is one of the busiest stretches of road in the Dallas–Fort Worth metroplex, at virtually all hours of the day and night.

The I-635 designation was truncated on December 2, 1971, when I-20 was rerouted south of Dallas, taking over 13 miles (21 km) of I-635's former route.

A $2.7-billion (equivalent to $3.61 billion in 2023[5]) project was started on May 16, 2011, to widen I-635 and dig subsequent high-occupancy toll lane tunnels beneath the primary roadway from I-35E to the High Five Interchange, a length of eight miles (13 km).