Asked to react to Mr Yeddyurappa declaring he would be sworn in on May 17, Mr Siddaramaiah quipped: "He is mentally disturbed."
The 69-year-old explained: "Even before election he has declared that he will take oath on this day...it looks like he is mentally disturbed."
Mr Yeddyurappa, the presumptive chief minister of the BJP, had said this morning, "I will fly to Delhi on the 15th once the results are announced and meet Prime Minister (Narendra) Modi. I will invite him and others for the swearing in ceremony which is going to happen most likely on the 17th."
The Chief Minister said Mr Yeddyurappa - the chief minister of the BJP's only government in the south in 2008 - was actually desperate because "Amit Shah, Modi, everybody knows BJP will not win."
Mr Siddaramaiah denied that he was struggling in both his seats after vacating his own constituency Varuna for his son. "In Badami, I will win with big margin. In Chamundeshwari also, I will win. I will win both constituencies," he claimed.
He also stressed that there is "no Modi wave".
He told NDTV: "Our party is not at all nervous. All leaders including Rahul Gandhi are very, very confident. There is a wave in favor of the Congress party."
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.