Extra Space Storage Inc. will be added to the the S&P SmallCap 600 Index after the close of trading on Monday, Dec. 31, the company said Thursday.

The company's listing will appear under the specialized sub-industry index for real estate investment trusts. According to Standard & Poor's, the SmallCap 600 Index covers about 3 percent of the U.S. equities market and is intended to be an efficient portfolio of companies that meet specific inclusion criteria to ensure that they are both investment-worthy and financially viable.

Extra Space Storage, headquartered in Salt Lake City, is a real estate investment trust that operates 644 self-storage properties in 33 states and Washington, D.C. The company is the second-largest operator of self storage in the United States.