Elon Musk is offering to buy Twitter for $43 billion in cash. 

Last week, Elon Musk became Twitter's largest shareholder. The Tesla and SpaceX CEO was going to join Twitter's board — then he wasn't.

Now the world's richest person is offering to buy the company for $43 billion — $54.20 per share — in cash, according to a regulatory filing posted Wednesday.

Musk's offer of $54.20 per share is 38% more than the value of Twitter stock the day before his investment was publicly announced, and it's 18.2% higher than Wednesday's closing price.

Twitter shareholder filed a securities fraud lawsuit against Musk, alleging his late disclosure of his stake in Twitter cost investors money and saved Musk around $143 million.

The company said its board "will carefully review the proposal to determine the course of action that it believes is in the best interest of the Company and all Twitter stockholders."

"My offer is my best and final offer and if it is not accepted, I would need to reconsider my position as a shareholder,"