Pappu Yadav, a Lok Sabha from Madhepura in Bihar, was on Monday sworn in by Speaker Somnath Chatterjee.
An accused in the murder of Communist Party of India-Marxist leader Ajit Sarkar, Yadav took the oath in Hindi.
Speaking during the Zero Hour, he charged the media with hounding him and his family.
Yadav was elected in a bypoll in October last year on a Rashtriya Janata Dal ticket after Railway Minister Lalu Prasad vacated the seat.
He was shifted from Beur Jail in Patna to Tihar Jail in Delhi following an apex court order.