Sushmita Dev, who joined Trinamool Congress a little over a month ago, was on Monday declared elected unopposed in the Rajya Sabha by-poll in West Bengal.
Dev collected the certificate of her election from the Assembly secretary and returning officer during the day, an official in the speaker's office said.
The BJP did not field any candidate against her in the by-poll which was necessitated as TMC's Manas Bhunia quit the Rajya Sabha after winning the assembly election from Sabang in Paschim Medinipur.
The saffron party leader Suvendu Adhikari had said that BJP would not field any canidate against her but concentrate in its fight to defeat Chief Minister Mamata Banerjee in the Bhabanipur assembly by-poll slated to be held on September 30.
Dev, daughter of former Congress heavyweight and union minister for several terms Santosh Mohan Dev, had quit the grand old party and joined TMC on August 16.
She later told reporters "I will intensify the battle against BJP, fight it in Rajya Sabha and voice the atrocities committed by the party's governments in Tripura and Assam. and accused the Narendra Modi government at the Centre of not allowing the opposition to air its views on important issues in Parliament.
Dev, a former Lok Sabha MP from Silchar in Assam, criticised the state Chief Minister Himanta Biswa Sarma for the firing on protestors during an eviction drive in Darrang district last week that claimed two lives.
She had lost to BJP in Silchar in the Assam assembly polls held earlier this year.