The spread of invasive mosquito species poses significant challenges to public health agencies worldwide. In particular, the genus Aedes possesses a number of species capable of transmitting lethal arboviruses. From 2014 to 2017, mosquitos were collected in western Illinois to assess the abundance and distribution of Aedes arboviral vectors. We found Ae. japonicus for the first time in Fulton and Schuyler counties, and confirmed active populations in Cass and McDonough counties. In September of 2016, we also found Ae. albopictus in Fulton and Cass County for the first time, highlighting its range expansion throughout Illinois. Discovery of these species suggests the potential for arboviral transmission to humans in western Illinois.