Singh, an independent MLA from Mokama, was administered oath by Speaker Vijay Kumar Chaudhary.
Immediately after taking oath, the three-time MLA was taken back by the police to Beur Central Jail where he has been lodged for the past six months.
The Patna High Court on March 17 allowed Singh's petition and asked the Assembly Secretariat to fix a date for his oath.
Singh faces several criminal cases and was arrested in July last year in connection with a kidnapping case in Bihta in Patna district in November, 2014.
After Anant Singh quit JD-U on September 2, 2015, the grand secular alliance fielded Member of Legislative Council, Niraj Kumar, from Mokama.