The seat fell vacant after M Venkaiah Naidu's election as the country's vice president on August 5.
Secretary of the legislative assembly Prithvi Raj said Mr Kannanthanam was elected unopposed as no other nomination was filed for the seat.
State parliamentary secretary Rajendra Rathore, social justice and empowerment minister Arun Chaturvedi, the BJP's state president Ashok Parnami and others were present during the occasion.
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