Ending days of speculation, JD-U state president Vashsisht Narain Singh made it clear that his party will contest the polls "along with the RJD to defeat the BJP".
"Seat sharing will be decided by the leaders of both parties. It is not a big issue," he said. "Our target is to defeat communal forces in Bihar."
On Saturday, the Election Commission said the by-elections will be held Aug 21 in Narkatiaganj, Rajnagar, Jale, Chapra, Hajipur, Mohiuddinnagar, Parbatta, Bhagalpur, Banka and Mohania.
The seats fell vacant due to the election of the legislators to the Lok Sabha in May. Six of the seats were held by the Bharatiya Janata Party, three by the RJD and one by the JD-U.
Narain Singh said the JD-U and RJD were confident of sweeping the assembly polls.
Last month, the RJD supported the JD-U's two candidates in the Rajya Sabha by-election.
The JD-U and RJD, which parted ways in 1994, are coming together following the BJP sweep in the Lok Sabha polls in Bihar.
An upbeat BJP has vowed to take power in Bihar on its own.