She had been arrested by the CBI in connection with the 2G Spectrum Case, which rocked the nation in 2011.
As the House met for the day, her name was called for taking oath or affirmation. Kanimozhi, who was re-elected to the Upper House in June, took oath in Tamil after which she greeted Chairman Hamid Ansari, who, with hands folded, remarked, "Welcome back" as members thumped their desks to welcome her.
She greeted senior leaders in the House before taking her seat.