OpenAI is suing Elon Musk over claims he has tried "nonstop" to slow down its business for his own benefit.
The company accuses the Tesla boss of using "bad-faith tactics" against OpenAI to help him control cutting-edge AI technology.
Mr Musk sued OpenAI chief executive Sam Altman last year in a bid to stop him from changing its corporate structure. He co-founded OpenAI with Mr Altman but left several years ago.
The countersuit opens up a new front in the high-stakes - and long-running - battle between two Silicon Valley heavyweights, who both say they are acting in the best interests of OpenAI and the public.