Tandon confirmed that she will be joining in the presence of SP chief Akhilesh Yadav.
She has also appealed to the people of her constituency Unnao to vote for the Samajwadi Party candidate in Bangarmau, where polling for by-elections will be held on Tuesday.
Congress General Secretary Ankit Parihar, who also quit the party last week, will be joining the Samajwadi Party.
The exodus from the Congress in Unnao continues, and so far, 42 party workers have resigned from Sadar in Unnao, four from Bhagwantnagar, 29 from Mohan, 24 from Bangarmau, 18 from Safipur and 12 from Purwa.
All of them are said to be ready to join the Samajwadi Party.