San Ygnacio is a census-designated place (CDP) in Zapata County, Texas, United States.

San Ygnacio, originally a Mexican pueblo, was founded in 1830 by settlers from nearby Guerrero, Tamaulipas on the outskirts of the recently established Uribeno Ranch.

Frequent Comanche attacks in the early days of San Ygnacio necessitated the use of defensive stone architecture which has endowed the present area with uncharacteristically enduring historical sites from that period.

San Ygnacio is also historically notable for its involvement in the short-lived revolution of the Rio Grande Republic, which was established at meetings which were convened in the town in 1839.

Although many of San Ygnacio's neighboring municipalities were evacuated and consolidated in the deliberate 1953 flooding of the region which created Falcon Lake and "New" Zapata, San Ygnacio's residents petitioned for the right to remain on their land, which was high enough to escape substantial damage.

The private River Pierce Foundation works to continue the restoration of the San Ygnacio Historic District.

[5] The community is also one of the few locations within the United States to view the Morelet’s seedeater, a small sparrow-like bird that inhabits second growth stands of Phragmites along the Rio Grande.

In the 2020 United States presidential election in Texas the community more heavily voted for Donald Trump than political operatives expected as residents felt obliged to protect their oil and gas related jobs, and pro-Trump organizers contacted residents in-person while pro-Biden ones, due to COVID-19-related cautions, did not.