I-37 begins in Corpus Christi on the Gulf Coast and heads north to San Antonio. It is the main route between I-35 and the Gulf of Mexico and the Rio Grande Valley cities of Brownsville and McAllen via US 281 and US 77. I-37 also functions as one of the few freeway hurricane evacuation routes for the southern Texas coast. I-37 roughly parallels US 181 and US 281 and in fact, US 181 both begins and ends at I-37.
The southern terminus of I-37 is at a junction with US 181 and SH 35 near the Corpus Christi Bay in Corpus Christi. It heads west from US 181 through Corpus Christi, intersecting two freeways, SH 286 (Crosstown Expressway) and SH 358 (Padre Island Drive). The highway turns towards the northwest after the SH 358 interchange roughly paralleling to the south of the Nueces River. Just prior to leaving the Corpus Christi city limits, I-37 intersects and has a short concurrency with US 77. US 77 merges with I-37 as a freeway from the south, the two continue to the north and split after crossing the Nueces River. I-37 continues to the northwest as US 77 continues to the northeast.
I-37 transitions to a rural setting once outside of the Corpus Christi city limits on its way to Mathis and Lake Corpus Christi. It continues on to the northwest and intersects US 59 east of George West. I-37 begins paralleling US 281 to the east before the two intersect and have a concurrency north of Three Rivers near Choke Canyon Reservoir. U.S. Route 281 Alternate splits off from I-37 near Sunniland and parallels I-37 before rejoining north of Campbellton. I-37 and US 281 remain concurrent until US 281 splits off to head to Pleasanton, while I-37 bypasses the city to the east. After US 281 leaves towards the northwest, I-37 turns to the north heading towards San Antonio.
As I-37 enters the San Antonio city limits, it intersects US 181 at the northern terminus of US 181. Continuing to the north, I-37 intersects I-410, the inner loop around San Antonio, at a stack interchange. At this junction, I-37 once again runs concurrently with US 281 which had been concurrent with I-410. Heading north through the south side of San Antonio, I-37 provides access to Brooks City-Base (formerly Brooks AFB). After a cloverleaf interchange at Loop 13, I-37 turns towards the northwest. The highway intersects I-10, which is concurrent with US 90 and US 87, at a stack interchange south of downtown. After the interchange, I-37 is once again heading north on the east side of downtown. It passes near the Alamodome, the Tower of the Americas, the River Walk and the Alamo. I-37 comes to an end northeast of downtown at a junction with I-35. US 281 continues to the north as a freeway providing access to the San Antonio International Airport. I-37 in San Antonio from I-10 to I-410 is designated the Lucian Adams Freeway, after World War II veteran Lucian Adams. Adams is a native of Port Arthur, and received the Medal of Honor for his service in France, along with the Bronze Star and Purple Heart for his gallantry during the Cassino Campaign.