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 the oath in Tamil, after which she greeted Chairman Hamid Ansari.
Ansari remarked with folded hands, "Welcome back" as members thumped their desks to welcome her.
She greeted senior leaders in the House before taking her seat.