This recent S&P 500 addition is now the best-performing stock in the index

A newcomer to America's most-watched index is now its top-performing stock of the year. Robinhood Markets  (HOOD) , which joined the index on Sept. 22, is now the index's top year-to-date performer.

Up 219% this year, the brokerage app now sits atop the S&P 500's year-to-date performers, surpassing Seagate Technology  (STX,)  which recently took the throne from one-time index leader Palantir Technologies  (PLTR) . Those two firms are up 164% and 142.7% year-to-date respectively; now the second and fourth-best performers in the S&P 500.

Related: This stock just displaced Palantir to become the S&P 500's top-performing stock

Robinhood's 2025 returns put it in a class matched by few other equities. Even more so when you zoom out and look at the last year: it's up 450% over the last 12 months. That strength was buoyed by a 45% year-over-year increase in revenue, which totaled $989 million in the latest quarter. During that time, it also more than doubled its net income.

The majority of its revenue came from transaction-based revenues, which amounted to $539 million in the latest quarter. Of that, 80% came from higher-risk options and crypto trading. A lesser share came from equity trades, "instant withdrawals, interchange revenues, and futures and prediction markets." 

A smaller sum came from net interest revenues, which amounted to $357 million, or 36.1% of the business's revenue. Those revenues were mostly comprised of margin interest, interest on segregated cash, securities, and deposits, and cash sweep. 

The remainder was sorted into Other Revenues (mostly their subscription service, Robinhood Gold), which came to $93 million.

The company's addition to the index came after being snubbed for other additions such as crypto competitor Coinbase  (COIN)  and brokerage company Interactive Brokers IBKR, among others. In total, 11 of the S&P 500's holdings have turned over this year.

And while it might be the case that Robinhood is the index's top-performer, it's also fair to point out that the company has not contributed as much to the index as other index high-flyers, mostly AI beneficiaries.

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Still, the company continues to execute and demonstrate promise, even after addition to the index. It remains to see whether it can maintain its growth despite that.

In recent weeks, it has announced a series of new features for traders, including forthcoming social features. And last week, it announced a foray into startup investing through a new business unit, Robinhood Ventures, which will allow retail investors access to the increasingly-congested private markets.