Officials said Friday that the girl from Venezuela was on a bus that left Brownsville, Texas.
She fell ill and died at a hospital in Salem, Illinois.
The bus trip was the latest organized by Texas and Florida to move migrants to democratically-controlled cities.
Details on how the girl died have not been released.
Newsom had blasted Abbott and Florida Gov. Ron DeSantis for allegedly coercing migrants on buses and planes, trains, porting them to cities thousands of miles away.