Dileep Verma, who is undergoing treatment at the district hospital and was shifted there from the jail, joined the ruling SP in front of local party office-bearers and media.
Speaking to reporters, Verma praised SP supremo Mulayam Singh Yadav and termed Congress as a sinking ship.
The former MLA has been in jail since June 2012 and is undergoing five years rigorous imprisionment in connection with an attack on a dalit constable in 1998.
His wife Madhuri Verma is a sitting Congress MLA.
A jail official, who did not want to be identified, feigned ignorance about Verma's joining SP in police custody against jail rules.