"Party president Akhilesh will be visting Patna on August 27 and attend the rally organised by Lalu Prasad," SP spokesman Rajendra Chowdhury said in Lucknow.
The rally is seen as an attempt to forge a larger alliance of like-minded parties to counter the BJP in the 2019 Lok Sabha elections.
Both SP and Bahujan Samaj Party have been invited by RJD chief Lalu Prasad for the rally.
But Mayawati's participation appeared uncertain as she distanced herself from posters showing her with non-BJP leaders.
The posters portrayed her alongside the cut-outs of other opposition leaders - Akhilesh Yadav, Tejaswi Yadav, Lalu Prasad, Sharad Yadav, Mamata Banerjee and Sonia Gandhi. It also carried pictures of some Dalit icons and the BSP logo. The poster also carried a message, "Samajik nyay ke samarthan mein vipaksh ek ho (Opposition must unite in the support of social justice)."
In her statement, Mayawati said, "The party does not have any Twitter account and, therefore, reports appearing in media based on a poster released on it are totally wrong and the BSP denies them."