ODM candidate Ochilo Ayacko has won the Migori Senate seat with 85,234 votes.
He won the seat after beating his rival Eddy Oketch of Federal Party of Kenya who garnered 60,555 votes.
The Independent Electoral and Boundaries Commission announced the results on Tuesday.
Voting closed at 5 pm on Monday in the 826 polling stations. The county has about 388,000 registered voters.
The by-election was, however, characterised by low voter turnout.
On Monday night, there was a standoff at the Migori county tallying centre after ODM supporters engaged in a shouting match with IEBC officials demanding that by-election results be displayed.
Before the chaotic scene, Ayacko was leading followed by Oketch. Police had to protect county returning officer Ruth Kulundu from angry agents and youths.
The seat fell vacant following the death of Senator Ben Oluoch at MP Shah Hospital in Nairobi June.