Roy was administered the oath of office and secrecy by assembly speaker Pranab Gogoi and she took it in Bengali.
She was accompanied by her husband, Minister Public Health and Engineering Gautam Roy and her son Rahul Roy, a former legislator from Algapur.
She thanked UPA chairperson Sonia Gandhi, Chief Minister Tarun Gogoi and Assam Pradesh Congress president Bhubaneswar Kalita for giving her an opportunity to become a member of the House.
Roy and her husband would be the first couple to be members of the same legislative assembly.
Meanwhile, Speaker Pranab Gogoi has convened an all-party meeting today as the budget session of the assembly begins tomorrow.