Peoria, Illinois. Peoria (/piËËÉËriÉ/, PEE-or-EE-uh) is a city in and the county seat of Peoria County, Illinois, United States, and the largest city on the Illinois River. Established in 1691 by the French explorer Henri de Tonti, Peoria is the oldest European settlement in Illinois, and is named after the Peoria tribe.