Mexicans looking to sneak into the United States have a brand new obstacle ... a gun-toting Steven Seagal ... who has just become an official border sheriff in Texas.

Seagal was sworn in as a sheriff's deputy in Hudspeth County, Texas this weekend ... and according to a sheriff's dept. spokesman, he'll be working full-time to help secure the border Texas shares with Mexico.

One of the sheriffs told Chron.com, "It became very clear to me that Mr. Seagal is not in this for the celebrity or the publicity ... He's like the rest of us that live down here, he has a sincere passion for his country and he wants to do more to help."

