Ellis announced earlier this month he would not seek re-election after more than four decades in state politics.
Roth is an Appleton Republican who served two terms in the Assembly before leaving in 2011. He released a statement late Monday announcing his decision to run.
Roth is the only Republican candidate who has expressed interest thus far in the seat in the Republican-leaning district.
Democratic Rep. Penny Bernard Schaber also is running for the seat.