By Frances Yoon

Younger people in South Korea have opened hundreds of thousands of trading accounts, hoping they will profit from turbulent markets to improve their otherwise lackluster economic prospects.

The novices are part of a global wave of individual traders, who have been lured into markets by prodigious volatility and with the encouragement of influential social-media users.

The new converts have helped cement South Korea's status as a major center for small investors, with roughly twice as many trading accounts per capita as the U.S.

This enthusiasm has helped fuel one of the biggest rebounds of any major equity market. As of July 3, the benchmark Kospi Composite Index had gained nearly 48% from a trough in mid-March.

Self-styled investing experts have popped up on YouTube and Instagram. Topics range from explaining the financial statements of index heavyweight Samsung Electronics Co. to breaking down "Bollinger bands" and other technical-analysis tools for a novice audience.

Park Jung-hyun, who goes by the screen name "Ho-du," has built a following of more than 163,000 people on YouTube after filming himself trading U.S. oil futures, with one subscriber calling him "the president of foreign futures." The 40-year-old influencer is sponsored by an online securities firm, and by a restaurant chain specializing in fried chicken, a hugely popular dish in South Korea.

On a typical live video, Mr. Park wears sunglasses and speaks into a webcam while trading natural-gas and oil futures. He attacks viewers who leave mean comments.

As the coronavirus pandemic was triggering big losses on the Kospi Composite in March, 31-year-old Choi Won-ki created his first investment account.

Using his smartphone, Mr. Choi said he quickly opened a trading account with NH Investment & Securities Co., a firm that has partnered with Kakao Corp., operator of a ubiquitous messaging app.

"It's a once-in-a-lifetime opportunity, and I didn't want to miss it," Mr. Choi said recently.

This is the latest in a series of investment frenzies, after South Korea embraced cryptocurrency trading and complex structured products. As in nearby China and Taiwan, the South Korean stock market is also one where individual investors are major players even in steadier times.

The Korea Financial Investment Association says trading accounts used by investors in their 20s and 30s jumped more than 50% year-over-year in the first quarter.

The nonprofit association doesn't provide a total number for these age groups, but says that overall, South Korean individuals had 31.25 million active trading accounts in April. That was up 13% in a year and is a record based on data going back to 2007.

For comparison, last year the Securities and Exchange Commission estimated that U.S. broker-dealers had about 102 million retail customer accounts. In 2018, the U.S. population was about 327 million, versus 51.6 million for South Korea, according to the World Bank.

Individual firms also report a jump in younger clients. At NH, people in their 20s and 30s accounted for 69.3% of the 610,000 new accounts opened from January through May 27.

A gloomy outlook helps explain the search for higher returns. Even before the pandemic, the Korea Economic Research Institute last year said the country had fallen into a low-growth, low-inflation era and warned growth could slow to 1% or lower by 2030.

The Bank of Korea expects the economy to shrink 0.2% this year and has cut its policy rate to a record-low 0.5%, helping push down interest rates on safer assets such as bonds.

"There is a depressing outlook that's gaining traction right now among younger generations. They think that the current state of the Korean economy is the best it's ever going to be and that it's all downhill from here," said Kim Taenam. Mr. Kim, who hosts a YouTube channel for new investors, pointed to demographic concerns about a low birthrate and shrinking population.

Kim Min-woo began trading oil after watching Mr. Park's videos. He said he bought West Texas Intermediate crude-oil futures in March, initially making 170 million won (about $142,000), before losing it all as oil crashed.

"I took a day off from work because I was devastated," he said. "I was thankful that I have a job, even though I get paid less than $2,000 a month."

Mr. Kim, a 26-year-old mechanical engineer, says he needs to keep investing. "It's not enough to just have a job anymore. My salary is similar to older colleagues. And I'm worried that I won't find good returns anywhere else. At this rate, I'll never be able to afford a house."

Some worry newer investors could get a nasty surprise. "It's concerning that the 20s and 30s age group, which has insufficient access to related information and limited investing experience, is jumping on the bandwagon into the stock market," said an official at the Korea Financial Investment Association. "The stock market is quite volatile with the spread of Covid-19."

In a video on YouTube posted June 29, Mr. Park lost $11,830 shorting 13 natural-gas futures contracts. Mr. Park says he is down about 160 million won for the year.

Others say the boldness is economically rational. "The younger generations are learning, some for the first time, what the consequences are of near-zero interest rates and slower growth that have been brought by the coronavirus, and that's really driving them to invest in assets with higher returns," said Hyun Choi, head of Korean equities at Barings.

Among other things, he said younger investors tended to invest in technology and health-care stocks despite those shares trading at higher valuation multiples than other sectors.

"They want returns," he said. "And they're aggressive and bold about the way they invest."

Write to Frances Yoon at frances.yoon@wsj.com