Tesla's CEO sold about 4.4 million shares on Tuesday and Wednesday, then another 5.2 million shares on Thursday, CNBC reported, citing Securities and Exchange Commission filings.
That left him with around 163 million shares in the electric-vehicle maker, or nearly 16% of the company, the Journal said. Musk is Tesla's largest shareholder and owned about 17% of the company before this week's sales.
After news of the sales started to break, Musk tweeted Thursday night: "No further TSLA sales planned after today."
Earlier this week, Twitter accepted Musk's offer to buy it for $54.20 per share for a total of about $44 billion, but the deal still requires shareholder and regulatory approval.
Tesla shares rose about 3 percent in Friday morning trading.