The National Institute of Anthropology and History says the tail is about 15 feet long and resembles that of a hadrosaur or crested duckbill dinosaur.
An institute Monday says it's not yet possible to confirm the species, but it would be the first full tail of that kind in Mexico.
Paleontologist Felisa Aguilar says they uncovered roughly half of the dinosaur, which was 36 feet long and lived about 72 million years ago.
The excavation took 20 days in the municipality of General Cepeda in the northern state that borders Texas.
The paleontologists, working with Mexico's National Autonomous University, also found hip bones.