Simon Property Group, Inc. is an American commercial real estate company in the United States and the largest real estate investment trust (REIT). It is also the largest shopping mall operator in America.

The company is a fully integrated real estate company which operates from five retail real estate platforms: regional malls, Premium Outlet Centers, The Mills, community/lifestyle centers and international properties. It currently owns or has an interest in more than 325 properties comprising approximately 241,000,000 square feet (22,400,000 m) of gross leasable area in North America and Asia. The company is headquartered in Indianapolis, Indiana and employs more than 5,000 people worldwide. It is publicly traded on the NYSE under the symbol SPG and is part of the S&P 100.

Simon Property Group has been the subject of several lawsuits and investigations regarding civil rights and discrimination.

Simon Property Group was formed in 1993 when the majority of the shopping center interests of Melvin Simon & Associates became a publicly traded company. Melvin Simon & Associates, owned by brothers Melvin Simon, Cowell Simon, and Herbert Simon, was founded in 1960 in Indianapolis, Indiana, and had long been one of the top shopping center developers in the United States.

In 1996, Simon DeBartolo Group was created when Simon Property merged with former rival DeBartolo Realty Corp. This was shortly after DeBartolo Realty became a publicly traded company encompassing the shopping mall interests of the Edward J. DeBartolo Sr. family, another leading developer. Simon DeBartolo rapidly acquired assets in the then-fragmented industry. Notable acquisitions included The Retail Property Trust and a group of properties held by IBM's pension plan in 1997 and Corporate Property Investors (CPI) in 1998. Following the CPI acquisition in 1998, the company announced it was reverting to its original name, Simon Property Group, as the DeBartolo family was resuming its private real-estate development operation, while retaining their interest in Simon.

Simon continued to be a prolific acquirer of shopping centers, including a portfolio from New England Development in 1999; several prime properties from Rodamco North America in 2002 (including Houston Galleria in Houston, Texas and SouthPark Mall in Charlotte, North Carolina); several high-profile properties such as Dadeland Mall, Fashion Valley Mall, Copley Place, Fashion Centre at Pentagon City, and Stanford Shopping Center; and in 2004, Chelsea Premium Outlets (now simply named "Premium Outlets"). In 2003, Simon became a co-owner of The Kravco Company, which became Kravco Simon. On April 3, 2007, a partnership including Simon agreed to acquire the Mills Corporation.

In June 2011, Simon Property Group entered into a partnership with Nintendo to provide complimentary 3DS Wi-Fi hotspots at nearly 200 of its malls. The Wi-Fi only works for Nintendo 3DS systems. It will not work on PC, Mac, iOS or Android.

In December 2013, Simon announced it would form a REIT of its smaller malls and community shopping centers called Washington Prime Group (Initially to be named SpinCo). The spin-off was created in May 2014 and was headed by Mark Ordan who was the final CEO of the Simon-bought Mills Corporation. The regional malls in WPG were still managed by Simon and flagged as Simon Properties on websites and inside their malls until early 2016, while Washington Prime managed the "strip centers" of the portfolio in-house. In September 2014, WPG announced to acquire Glimcher Realty Trust and its properties, in which Washington Prime Group would be renamed WP Glimcher when the deal was made. The deal was completed in January 2015. As part of the deal, Simon acquired Jersey Gardens (renamed to "The Mills at Jersey Gardens" and added to the Mills portfolio) in Elizabeth, New Jersey and University Park Village in Fort Worth, Texas, while WP Glimcher acquired Brunswick Square in East Brunswick, New Jersey from Simon.

On November 29 of 2009, Financial Times reported that Simon may have attempted to acquire its failing rival General Growth Properties, which was operating under Chapter 11 bankruptcy protection. Should GGP have been acquired in its entirety, such deal would be worth up to $30 billion at the time. Simon hired property investment firm Cohen & Steers, J.P. Morgan, as well as the Lazard investment bank and the Wachtell Lipton Rosen  &  Katz law firm to explore the possibility of acquiring GGP.

On February 16, 2010, Simon announced that it placed a bid on February 8 to acquire General Growth Properties in a deal worth $10 billion. However, the bid was rejected by General Growth twice during the week it was announced. On February 19, 2010, one GGP shareholder filed suit (Young v. Bucksbaum) against the company's board of directors for rejecting Simon's bid, accusing chairman John Bucksbaum and six other board members of breaching their fiduciary duty to GGP's investors. The General Growth board favored an investment offer from Brookfield Asset Management worth $2.6 billion.

Simon's next step, on April 14, 2010, was to announce a $2.5 billion equity investment offer which equaled the price per share of Brookfield's offer. Simon claimed that the deal was more favorable to GGP and its equity holders than Brookfield's offer, stating that it would eliminate the highly dilutive warrants that GGP would issue to Brookfield, Pershing Square and Fairholme Capital. Simon's offer also included a co-investment commitment by Paulson & Co worth $1 billion. Simon Property Group had not ruled out a full takeover of General Growth, claiming that their investment offer would give them more time to work out their differences concerning antitrust issues.

On May 7, 2010, Simon Property Group decided to withdraw its acquisition and recapitalization proposals for General Growth Properties.

On December 8, 2009, Simon Property Group was offered the sale of Prime Retail's Prime Outlets portfolio for $2.24 billion, which included centers at Williamsburg, Virginia, San Marcos, Texas and Hagerstown, Maryland. In May 2010, Simon surprisingly acquired Prime Outlets-Puerto Rico in Barceloneta, which was renamed "Puerto Rico Premium Outlets." However, at the time, Simon did not acquire the whole portfolio, excluding Prime Outlet's center at St. Augustine, Florida where Simon's Premium Outlet mall was already located. Finally, on August 30, 2010, Simon officially acquired the Prime Retail portfolio of twenty-one outlet malls, including the Barceloneta, Puerto Rico location acquired in May 2010. Three properties were not included: the indoor St. Augustine, Florida property, and developments at Grand Prairie, Texas and Livermore Valley, California. The acquired properties received Premium Outlet branding just after the completion of the deal. The developments at Grand Prairie, Texas and Livermore Valley, California were acquired in 2012, just months after their openings, and were rebranded as "Premium Outlets."

At the end of 2010, Simon Property Group attempted to acquire UK-based retail investment group Capital Shopping Centres (CSCG), after CSCG signed a deal with Peel Group to acquire the Trafford Centre. This offer was later declined by Capital Shopping Centres shareholders in an EGM, with the shareholders, many of whom are South African, voting instead for the deal with the Peel Group.

On August 1, 2013, Toronto Premium Outlets opened in Halton Hills, Ontario, Canada. Premium Outlets Montreal, which is the second Premium Outlet Center located in Canada opened in October, 2014.