The US Centres for Disease Control and Prevention confirmed the child's death reported on Wednesday is the first measles death in the country since 2015.
The death was a "school-aged child who was not vaccinated" and had been admitted to hospital last week, the Texas Department of State Health Services said on Wednesday in a statement.
The measles outbreak in rural West Texas has grown to 124 cases across nine counties, the state health department said Tuesday.
An unvaccinated child has died from measles in West Texas, the first death in an outbreak that broke out late last month.
Gaines County, which has 80 cases, has one of the highest rates in Texas of school-aged children who opt out of at least one required vaccine, with nearly 14% of children from kindergarten to the age of 16-17 in the 2023-24 school year.