These Were the 3 Top-Performing Stocks in the S&P 500 in September 2025 -- and One Popped 68%!

In September, while the S&P 500 gained a solid 3.5%, Warner Bros. Discovery (WBD), AppLovin (APP), and Western Digital (WDC) significantly outperformed, surging 68.1%, 49.2%, and 46.6% respectively. WBD's rally was fueled by plans to split and buyout rumors, APP benefited from its app monetization software success and recent S&P 500 inclusion, and WDC capitalized on robust demand for data storage driven by cloud computing and AI, alongside positive analyst revisions.

Analysis

In September, three specific equities—Warner Bros. Discovery (WBD), AppLovin (APP), and Western Digital (WDC)—delivered exceptional returns, significantly outperforming the S&P 500's solid 3.5% gain. Warner Bros. Discovery's 68.1% surge appears primarily event-driven, fueled by news of a planned corporate split and unsubstantiated buyout rumors, rather than a shift in underlying fundamentals. AppLovin's 49.2% increase was supported by both strong business performance in app monetization and a significant technical catalyst from its recent inclusion in the S&P 500 index. The company is also noted for its expansion beyond gaming applications and strong international presence. Similarly, Western Digital's 46.6% rise was propelled by powerful secular tailwinds, as the hard disk drive specialist benefits from robust data storage demand driven by the growth of cloud computing and artificial intelligence. This positive industry outlook was validated by several investment banks, which raised their price targets for WDC during the month.

Key Decisions for Investors

• Given the magnitude of the September rallies, investors should be cautious about extrapolating similar monthly gains and re-evaluate valuations for WBD, APP, and WDC.
• The rally in Warner Bros. Discovery is highly speculative; positions should be monitored based on concrete news regarding its proposed corporate split or any material M&A developments, not just rumors.
• For AppLovin, investors should assess its ability to sustain growth and monetization momentum following the one-time catalyst of its S&P 500 inclusion.
• For Western Digital, the key consideration is the durability of the AI- and cloud-driven demand cycle for data storage, as this underpins the positive analyst revisions and recent stock performance.