The by-poll was necessitated due to the alleged suicide of former Chief Minister Kalikho Pul.
North East Democratic Alliance (NEDA) candidate Dasanglu Pul of BJP won the Hayuliang Assembly by-poll after defeating her lone rival Yompi Kri, an Independent candidate, by 942 votes.
Ms Pul, who was contesting on a BJP ticket, polled 5,326 votes while Ms Kri could secure 4,384 votes in the by-poll held on November 19.
The winner is the youngest of three wives of former Arunachal Pradesh Chief Minister Kalikho Pul. The by-poll was necessitated due to his alleged suicide.
A total of 108 voters opted for NOTA , Chief Electoral Officer Kaling Tayeng said, adding an estimated 72 per cent electorates exercised their franchise in the election.
The strength of BJP, alliance partner of PPA-led NEDA government, has now risen to 12.
People's Party of Arunachal (PPA) has 43 MLAs, Congress three and Independents two in the 60-member House.