The billionaire tech giant just filed suit against OpenAI -- the company he helped start/finance back in 2015 -- alleging CEO Sam Altman and President Greg Brockman have twisted the org's original mission of humanitarianism into nothing but a profit-seeking venture.

In the docs, obtained by TMZ, Musk says he, Altman, Brockman, and others established a founding agreement for OpenAI at its launch -- a constitution of sorts, if you will -- which EM claims explicitly stated they were doing all this for the benefit of all people ... not for profit.

He also claims that another principle they came to was having their code open and available to the public ... so people could understand and collaborate on the development of the technology.

Elon says in his suit that this founding agreement they wrote out was actually memorialized as the north star of their new company -- but last year, he claims the remaining OpenAI honchos torched when they rolled out their latest version of ChatGPT.

Of course, he's referring to GPT-4 ... which Elon and others have called the smartest iteration of AI yet -- and Elon claims Microsoft even paid a ton of money for an exclusive licensing agreement with OpenAI for the use of some of the technology.

The problem, Elon claims, is that the guts of how GPT-4 came together were completely hidden ... which he says is a gross departure from the commitment to openness and transparency to the public laid out in the founding agreement almost 10 years ago.

Elon also points to the whole episode of Altman getting booted from the OpenAI board and then brought back in days later with Microsoft-friendly board members as more proof that OpenAI has clearly bucked its original mission ... and is now simply chasing profit.