Alam joined Lalu Prasad's party in presence of senior RJD leaders including its national vice-president Shivanand Tiwary at RJD's office here.
He accused Chief Minister Nitish Kumar's party JD(U) of "betraying secular forces" by joining hands with the BJP.
The development came a day after bypolls were announced for two assembly segments in Bihar, besides the Araria Lok Sabha seat which was represented by Alam's late father Mohammad Taslimuddin, who was a RJD MP.
Alam told reporters "I had joined the JD(U) as it was then a part of the Grand Alliance which represented secular forces. Ever since the party betrayed the secular forces, I have been under pressure from my voters and my mother that I join the party to which my father belonged".
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Asked whether he has been promised the RJD ticket from Araria, Alam - who had met former Chief Minister and party supremo Lalu Prasad's wife Rabri Devi earlier in the day - said "the decision shall be taken by the party".
Tiwary said "the development marks a growing discontentment with Nitish Kumar's capitulation before the BJP. No longer does the Chief Minister enjoy the upper hand like he used to in the past. More such developments are in the offing as has been indicated by outbursts of Jitan Ram Manjhi".
The RJD vice-president was referring to the statement of Manjhi, the founder of Hindustani Awam Morcha (HAM), earlier in the day that his party be allowed to contest the by-poll from Jahanabad assembly segment which has fallen vacant upon the death of RJD MLA Mundrika Yadav.