Interstate 19 (I-19) is a north–south Interstate Highway located entirely within the US state of Arizona. I-19 travels from Nogales, roughly 90 meters (300 ft) from the Mexican border, to Tucson, at I-10. The highway also travels through the cities of Rio Rico, Green Valley, and Sahuarita.

Having a total length of just over 102 kilometers (63 mi), I-19 is the eighth-shortest primary (two-digit) Interstate Highway in the contiguous 48 states, where only I-86 (Idaho), I-11 (Nevada), I-2 (Texas), I-42 (North Carolina), I-14 (Texas), I-97 (Maryland), and I-87 (North Carolina) are shorter.

While the highway is short, it is a very important corridor, serving as a fast route from Tucson and Phoenix (via I-10) to the Mexican border. The highway is a portion of the US section of the CANAMEX Corridor, a trade corridor that stretches north from Mexico across the US to the Canadian province of British Columbia.

In Nogales, the southern terminus of I-19 is at West Crawford Street, adjacent to the international port of entry, and southbound travelers can continue into Heroica Nogales, Mexico, via state-maintained surface roads, and connect with Federal Highway 15 (Fed. 15) either to the south or west of Nogales, Sonora.

Starting from the southern terminus at kilometer post 0 (not milepost 0), I-19 initially heads briefly south then west on surface streets, navigating its way through the town of Nogales for 0.32 kilometers (0.2 mi) before becoming an Interstate-grade freeway and making the turn to head north toward Tucson. It has interchanges with two other state highways near the southern end of the route, State Route 189 (SR 189) at exit 4 and SR 289 at exit 12. The interchange with SR 189 at exit 4 both serves to funnel traffic so as to bypass around Nogales and Heroica Nogales for travelers bound to or from Hermosillo or Mexico City and provides for the continuous flow of freight and truck traffic through the larger Nogales-Mariposa Port of Entry to Fed. 15, which has its northern terminus at the US–Mexico border with SR 189 and its southern terminus 2,179 kilometers (1,354 mi) away in Mexico City. After exiting Nogales to the north, I-19 passes near and around a series of sparsely populated towns and retirement communities along the banks of the Santa Cruz River, including Rio Rico, Tubac, Amado, Green Valley, and Sahuarita. For several miles near Amado and Green Valley, the eastward view from I-19 provides scenic views of Madera Canyon and the Santa Rita Mountains in the Coronado National Forest.

Just before entering Tucson, I-19 passes through the eastern section of the San Xavier Indian Reservation where it makes its only crossing of the Santa Cruz River. As I-19 enters the Tucson city limits, it has an interchange with SR 86 at exit 99 before reaching its northern terminus at an interchange with I-10.

Nearly the entire route of I-19 follows, or is adjacent to, the former routing of US Route 89 (US 89) and the Santa Cruz River, which flows northward from Mexico, through Tucson and usually disperses into the desert between Marana and the Gila River, southeast of Phoenix. Most of the time, much of the river is dry, but heavy storms can cause it to overflow its banks, flooding farmland before reaching the Gila River.

I-19 is unique among US Interstates because signed distances are given in meters (hundreds or thousands as distance-to-exit indications) or kilometers (as distance-to-destination indications), and not miles. However, the speed limit signs give speeds in miles per hour. According to the Arizona Department of Transportation (ADOT), metric signs were originally placed because of the push toward the metric system in the US at the time of the original construction of the highway.