Avinash is the son of Umesh Jadhav, whose resignation on March 3 from the seat as a rebel Congress legislator necessitated the by-election.
Jadhav senior quit the Congress and joined the BJP in early March and defeated Mallikarjun Kharge in the Gulbarga (SC) Lok Sabha seat on Thursday.
The Congress, however, retained the Kundgol Assembly segment with its candidate Kusumavati Channabasappa defeating S.I. Chikkanagoudar of the BJP by a narrow margin of 1,601 votes.
Death of Congress Minister for Municipalities C.S. Shivalli on March 22 necessitated the bye-poll.
A three-time legislator, Chikkanagoudar lost to Shivalli by a margin of 633 votes in the May 2018 state Assembly elections.
Kundgol is in the state's northwest Dharwad district, about 440km from Bengaluru.