Deputy Chief Electoral Officer D J Bhattacharjee said Wangsu polled 5,231 votes and Maham 4,346 votes.
Journalist-turned-politician, Wangsu had contested unsuccessfully from the constituency in 2009 and 2014 as a People's Party of Arunachal candidate and an Independent respectively.
The bye-election was held following the death of sitting Congress MLA Newlai Tingkhatra at Gurgaon on May 7, a month after the simultaneous state Assembly and Lok Sabha elections in the state and nine days before counting of votes.
Tingkhatra had won the seat posthumously. With Wangsu winning from the constituency, Congress has 42 MLAs now in the 60-member Assembly, while Opposition BJP has 11 legislators.
Peoples Party of Arunachal has five members. There are two Independents.