Now, shares of blank-check companies almost always go up. The last 140 SPACs to go public have either logged gains or ended flat on their opening day of trading, per a Dow Jones Market Data analysis of trading in blank-check companies through Thursday. One hundred and seventeen in a row have risen in their first week. The gains tend to continue, on average generating bigger returns going out to a few months.

The gains in companies that don’t yet have any underlying business underscore the wave of speculation in today’s markets. Merging with a SPAC has become a popular way for startups in buzzy sectors to go public and take advantage of investor enthusiasm for futuristic themes.

But lately, day traders are even putting money into SPACs before they have revealed what company they are buying. At that stage, they are pools of cash, so investors are wagering that the company will eventually complete an attractive deal.

Despite the risks, many are embracing the trade, underscoring how online investing platforms and social-media groups now send individuals flocking to new corners of markets, including shares of unprofitable companies such as GameStop and

AMC Entertainment Holdings Inc.

AMC 53.65%

That trend also is playing out in everything from shares of silver miners to SPACs, which were relatively rare before last year but are suddenly ubiquitous in finance.

“I would just have a bad case of FOMO if I wasn’t in SPACs,” said

Marco Prieto,

a 23-year-old real-estate agent living in Tucson, Ariz., referring to the fear of missing out that is driving many individuals to put money into markets.

He has a roughly $50,000 portfolio and about 60% of his holdings tied to blank-check companies. Some of his positions are early on in shell firms such as

Social Capital Hedosophia Holdings Corp. VI,

while others are based on rumors tied to possible deals by companies including

Churchill Capital Corp. IV.

Shares of that company have more than doubled since Bloomberg News reported on Jan. 11 that it is in talks to combine with electric-car firm Lucid Motors Inc. Trading got so frenzied that the SPAC put out a statement a week later saying it wouldn’t comment on the report and that it is always evaluating a number of possible deals. The stock has still been gyrating in the days since.

Investors betting on SPACs even before such reports is extraordinary because the underlying value of a blank-check firm before it pursues a deal is the amount of money it raises for a public listing. That figure is typically pegged at $10 a share. Still, it has become common for investors to buy at higher prices such as $11 or $12 to back big-name SPAC founders such as venture capitalist

Chamath Palihapitiya

and former Citigroup Inc. deal maker

Michael Klein.

In another sign blank-check firms are now frequently traded by individuals, several SPACs and companies that have merged with them recently joined GameStop and AMC on a list of stocks that had position limits on Robinhood Markets Inc., a popular brokerage for day traders. Those restricted included Mr. Klein’s Churchill Capital IV and a few of Mr. Palihapitiya’s SPACs in the

Social Capital Hedosophia

SPCE 2.74%

franchise.

The flood of money pouring in is a concern for skeptics who worry that everyday investors don’t understand the dangers of the trade. Even recent losses in a few hot companies such as electric-truck startup

Nikola Corp.

NKLA -0.39%

and health-care firm MultiPlan Inc. that merged with blank-check firms aren’t deterring investors because of the gains in other SPACs.

“It’s a tremendous amount of speculation,” said

Matt Simpson,

managing partner at Wealthspring Capital and a SPAC investor. His firm invests when SPACs go public or right after, then takes advantage when shares rise and typically sells before a deal is completed. He advertised an expected return from the strategy of 6% to clients, but last year it returned 20%.

Ninety-one SPACs have raised $25 billion so far this year, putting the market on track to shatter last year’s record of more than $80 billion, according to data provider SPAC Research.

Fast gains in the shares can result in big payoffs for their founders and the first investors in blank-check firms like Mr. Simpson. These earliest investors always have the right to withdraw their money before a deal goes through. The traders who get in later don’t have those same privileges, but that hasn’t been a deterring factor.

“If you don’t take a risk, there’s really no opportunity at all,” said

Chris Copeland,

a 36-year-old in upstate New York who started day trading on the platform Robinhood with his girlfriend last month. Roughly three-quarters of his portfolio is tied to SPACs such as

GS Acquisition Holdings Corp. II.

Trading volumes in many popular blank-check firms have increased lately, an indication of investors’ heightened activity. That trend is even drawing attention from some SPAC founders.

“It worries me,” said veteran investor and SPAC creator

Bill Foley.

Trading volumes have surged in one of the SPACs founded by the owner of the Vegas Golden Knights hockey team, especially since it announced a $7.3 billion deal to take

Blackstone Group Inc.

BX 0.21%

-backed benefits provider Alight Solutions public last week.

One reason traders are getting into blank-check firms when they are just pools of cash is that the time it takes for a SPAC to unveil a deal has dwindled. Blank-check firms normally give themselves two years to acquire a private company, but many these days need only a few months.

It also doesn’t take long for investor speculation about a blank-check firm’s acquisition to build, particularly because SPACs can indicate the sector in which they hope to complete a deal.

Excitement can be triggered by a SPAC pioneer like Mr. Palihapitiya, who sometimes hints to his more than 1.2 million Twitter followers when activity is coming. The former Facebook Inc. executive took space-tourism firm

Virgin Galactic Holdings Inc.

public in 2019 and last month reached a deal with Social Finance Inc.

Even though he invests in a number of blank-check firms other than his own—often when SPACs need to raise more money to complete deals—shares of his own companies can climb following such tweets. One example came Jan. 21, when one of his blank-check firms rose about 4% after Mr. Palihapitiya started a tweet by saying “I’m finalizing an investment in ‘???.’”

The SPAC has since given back those gains after no news about an acquisition came out and it was revealed that Mr. Palihapitiya’s investments were in companies unrelated to his own. He declined to comment.

Mr. Palihapitiya also has thrown himself into the frenzy of activity around GameStop trading, publicizing an options trade last week in the stock and taking profits on it.

Reports about possible mergers like those surrounding the Churchill Capital IV SPAC and a possible combination with Lucid Motors also quickly attract hordes of buyers. That blank-check firm is now owned by many individuals, including Messrs. Prieto, Copeland and

Jack Oundjian,

a 40-year-old who lives in Montreal.

“I’m very excited that we have a chance to be able to participate in what could be future unicorn companies,” or startups valued at $1 billion or more, Mr. Oundjian said. He said he views SPACs as long-term investments rather than fast trades, and holdings tied to the sector make up about 30% of his roughly $1.2 million portfolio.

Write to Amrith Ramkumar at amrith.ramkumar@wsj.com