S&P Dow Jones Indices announced Monday evening that SanDisk Corp. will be added to the S&P 500 Index this Friday. Following the news, shares of the computer storage device manufacturer surged over 9% in after-hours trading. The company will transition from the S&P SmallCap 600 Index to the S&P 500, replacing advertising and marketing firm Interpublic Group of Companies Inc.

The index adjustment will take effect at Friday's market open, with U.S. stock trading hours shortened due to the Thanksgiving holiday.

As of Monday, SanDisk's market capitalization stood at approximately $34 billion, making it an outlier in the small-cap index due to its significantly larger size compared to other constituents. SanDisk also edged out Strategy Inc., an enterprise software company that claims to be the largest corporate holder of cryptocurrency. Strategy, formerly known as MicroStrategy, recently became eligible for inclusion in the S&P 500.

Monday's announcement came earlier than some investors had anticipated. Market expectations had pointed to early December for the index rebalancing that would include either SanDisk or Strategy. However, the index provider retains discretion over the timing and execution of such adjustments, or even the decision to forgo them entirely, as seen in June.

SanDisk's stock has surged nearly 400% over the past three months, fueled by the AI boom.