Elon Musk’s sale of roughly $5 billion in Tesla Inc. shares in recent days is likely just the start of his selling wave.

When the Tesla chief executive took to Twitter over the weekend, pledging to unload 10% of his stock in the electric-vehicle company he runs, he owned roughly 170 million shares. Mr. Musk has since sold about 4.5 million shares through Wednesday, according to regulatory filings.

To...

Elon Musk’s sale of roughly $5 billion in Tesla Inc. shares in recent days is likely just the start of his selling wave.

When the Tesla chief executive took to Twitter over the weekend, pledging to unload 10% of his stock in the electric-vehicle company he runs, he owned roughly 170 million shares. Mr. Musk has since sold about 4.5 million shares through Wednesday, according to regulatory filings.

To make good on his Twitter pledge, Mr. Musk likely would need to sell more than 12.5 million additional shares, valued at more than $13 billion at today’s close.

The precise number depends on how he defines his ownership stake. Mr. Musk didn’t respond to a request for comment.

Mr. Musk’s sales this week rank among the largest-ever by a chief executive over a few days, according to research firm Equilar Inc. Amazon.com Inc. founder Jeff Bezos earlier this year—before relinquishing his CEO title—sold 2 million shares in his company over little more than a week for proceeds of roughly $6.6 billion, according to Equilar.

Tesla shares have slumped in the wake of Mr. Musk’s Saturday poll, declining by roughly 13% through Thursday, when the stock closed at $1063.51. The stock retreated 0.4% Thursday, the third of four trading days this week when it ended down. Despite that decline, Tesla stock is trading near record highs, with the company valued at more than $1 trillion.

Tesla investors may need to brace for more turmoil in the stock, said Ben Silverman, director of research at InsiderScore, which tracks executive-trading data.

“I don’t think the volume of Musk’s transactions will create volatility. I think volatility comes from the interpretation of his actions and his own words,” Mr. Silverman said.

Mr. Musk has long been reluctant to sell Tesla stock. Selling shares could weaken his control over the company. Unlike Meta Platforms Inc. (formerly Facebook Inc.) and Google parent Alphabet Inc., Tesla lacks a dual-class of stock ownership that gives founders supervoting power over common shareholders. Mr. Musk sold nearly $600 million worth of stock in 2016 to pay taxes, Equilar data show.

Mr. Musk—the world’s richest person, according to the Bloomberg Billionaires Index—remains Tesla’s largest investor by far, even after the stock sales that have been disclosed in regulatory filings.

Mr. Musk, who exercised just over 2 million stock options on Monday, faces an August 2022 deadline to exercise more than 20 million additional options or let them expire worthless.

Further stock sales could go a long way toward paying a massive tax bill that Mr. Musk could owe upon exercising those vested options. The difference between the value of Tesla’s stock when Mr. Musk exercises his options and the exercise price of $6.24 will be taxable income.

Mr. Musk reported selling more than 900,000 shares on Monday, worth a total of roughly $1.1 billion, to cover tax withholding obligations.

Monday’s option exercise and sales were made under a preset trading plan Mr. Musk established on Sept. 14, according to regulatory filings, almost two months before he raised the idea of a sale on Twitter. Such trading arrangements, called 10b5-1 plans, are designed to enable company insiders to sell based on a set schedule, price triggers or other factors without running afoul of insider-trading rules.

The filings disclosing Mr. Musk’s subsequent sale of roughly 3.6 million shares spanning Tuesday and Wednesday don’t include the same footnotes about preset trading plans and tax withholding obligations.

On Saturday, Mr. Musk framed the idea of a share sale in terms of a continuing debate about how some of America’s wealthiest individuals should be taxed.

Mr. Musk doesn’t accept a cash salary from Tesla. In a tweet Saturday, he said, “the only way for me to pay taxes personally is to sell stock.”

The options expiring in August are part of a 2012 pay package. Since then, Mr. Musk has secured additional options. Exercising them could trigger volatility down the road. Mr. Musk’s next major deadline to exercise a large number of options looms in 2028, according to a Tesla regulatory filing, though he faces 2023 deadlines to exercise some vested options.

Write to Rebecca Elliott at rebecca.elliott@wsj.com