BJP's Ramji Lal Sharda and Union Tourism Minister Subodh Kant Sahay's brother Sunil Sahay of the Congress finished third and fourth respectively, according to official sources here.
The bypoll, which was held on June 12, was necessitated following the death of Congress MLA Gopal Sharan Nath Shahdeo.
The win helps the AJSU increase its assembly strength to six from the present five MLAs.
The party, which is led by Jharkhand Deputy Chief Minister Sudesh Mahto, had only two MLAs till 2005.
Launched as All Jharkhand Students Union to fight for separate state, the Election Commission has allowed it as a political party in its abbreviated form- AJSU party.
With its candidate Ramji Lal Sharda defeated, the ruling BJP failed to become the single largest party in the house. The saffron party has strength equal to the JMM with 18 MLAs each in the 82-member house.
Sharda, who was a former minister in the erstwhile Babulal Marandi government, had also lost the 2009 assembly polls to Congress candidate by a slender 25 votes.