There’s a new high-flying stock on Wall Street that some investors are eagerly piling into. Its largest shareholder is former President Donald J. Trump.

His social media company, Trump Media & Technology Group, began trading on the Nasdaq on Tuesday, under the ticker DJT, and immediately surged in value, gaining 40 percent in early trading.

Trump Media — the parent of Truth Social, the online platform that is Mr. Trump’s main megaphone for reaching supporters and going after critics — closed its merger with a cash-rich public shell company on Monday.

The shell company’s stock had jumped higher ahead of the deal, in a frenzied trade that has fueled the company since it proposed the merger with Mr. Trump’s firm in 2021. Trump Media has an estimated market value well over $6 billion, more than established corporations like Alaska Airlines, Western Union and American Eagle Outfitters.

The biggest beneficiary of the market action is Mr. Trump, who owns about 60 percent of Trump Media’s shares. His stake in the company is worth more than $4 billion.

Before the merger, shares of the shell company — Digital World Acquisition Corporation — had long behaved as something of a proxy for investor sentiment about Mr. Trump. And that is likely to continue for the merged companies, especially as Mr. Trump remains in the headlines with pending trials and the presidential campaign.