Robinhood Markets Inc. unveiled the paperwork for its long-awaited initial public offering Thursday, detailing how the flood of everyday investors into the stock market has supercharged the trading app’s growth.

Robinhood, which plans to list on Nasdaq under the ticker symbol HOOD, generated $522 million of revenue in the first quarter, mostly from trading activity, more than quadruple its level from the first quarter of 2020. More than $4 out of every $5 Robinhood earned in first-quarter revenue stemmed from payments it received from high-speed trading firms to which it routed customers’ stock, option and cryptocurrency trades, a controversial practice known as payment for order flow.

The number of funded accounts at Robinhood swelled to 18 million at the end of March, more than double their number from a year earlier, as everyday investors signed up in droves to participate in rallies in meme stocks such as GameStop Corp. and cryptocurrencies like dogecoin.

Despite the increase in users and trading-based revenue, Robinhood reported a first-quarter loss of $1.4 billion.

The first-quarter loss was largely due to a $1.5 billion one-time charge, related to an emergency fundraising in late January at the height of the GameStop rally. The clearinghouse that processes and settles Robinhood’s trades asked the company to put up billions of dollars in extra collateral to cover potential losses on volatile trades, prompting Robinhood to restrict trading in certain highflying stocks until it could complete a sale of convertible notes.

Robinhood is set to be the buzziest company to tap the U.S. IPO market this summer, and it enters one of the most welcoming markets in years. Volatility is low, and major stock indexes are at or near records. This past week was the biggest of the year for traditional IPOs as measured by money raised, according to Dealogic.

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Bankers, investors and lawyers say this is shaping up to be the busiest year in decades. U.S.-listed IPOs have raised more than $190 billion so far in 2021, according to Dealogic, already surpassing 2020’s record-breaking total. While more than half of the money raised has been for special-purpose acquisition companies, or SPACs, traditional IPOs have raised more than $85 billion and are still on track to raise the most money ever in a year.

Robinhood’s filing revealed the company is still dealing with the customer-service and regulatory fallout from its January trading restrictions.

Users opted to transfer roughly 206,000 Robinhood accounts that held $4.1 billion, or about 5% of the company’s assets under custody, to other brokerage firms in the first quarter. In an average quarter in 2020 for Robinhood, only about 22,000 accounts put in transfer requests.

The company, its subsidiaries and Chief Executive Vlad Tenev have received requests for information and some subpoenas related to the trading restrictions from the Justice Department, the Securities and Exchange Commission, and the New York attorney general’s office, among other entities. The San Francisco U.S. attorney’s office executed a search warrant to obtain Mr. Tenev’s cellphone, Robinhood said.

Despite agreeing this week to settle a wide-ranging investigation into a host of its business practices for nearly $70 million, Robinhood also faces a number of federal and state investigations. Those involve trading outages that hit its platform in March 2020, its handling of user account takeovers, its practices around approving users to trade options and its compliance with anti-money-laundering rules, among other matters. Robinhood is currently negotiating a settlement with New York’s top financial regulator that would include a monetary penalty of at least $10 million.

Write to Peter Rudegeair at Peter.Rudegeair@wsj.com and Corrie Driebusch at corrie.driebusch@wsj.com