Square is a point-of-sale system for sellers with physical or online stores. Launched in 2009 by Block, Inc., it enables merchants to accept card payments and manage business operations. As of 2024, Square is the U.S. market leader in point-of-sale systems, serving 4 million sellers and processing $228bn annually.

Square is cloud-based and offers both physical devices, which read payment card information, and software. It offers financial services and includes features designed to support business operations. Square provides e-Commerce and inventory capabilities, customer appointments, payroll processing, shift scheduling, and access to banking and business loans. As of 2024, Square is available in the United States, Australia, Canada, France, Japan, Republic of Ireland, Spain and the U.K.

The inspiration for Square occurred to Jack Dorsey in 2009, when his friend Jim McKelvey was unable to complete a $2,000 sale of his glass faucets and fittings because he could not accept credit cards. In December 2009, Dorsey announced the launch of Square, which allowed merchants and individuals to accept secure payment from credit and other cards using a mobile phone and a "card-reading dongle".

At the TechCrunch Disrupt conference in May 2011, Square announced the release of two apps, Square Card Case (later rebranded Square Wallet) and Square Register. Square Wallet, before it was removed from the Apple App Store and Google Play Store in 2014, allowed customers to set up a tab and pay for their order by providing their name (or a barcode) using a stored credit, debit, or gift card.

In April 2012, rival payment company Verifone claimed that the Square system was insecure and that a reasonably skilled programmer could write a replacement app that could use the Square device to skim a credit card and return its details, because of the lack of encryption. VeriFone posted a demonstration video and sample skimming app to its web site. Dorsey called VeriFone's claims "neither fair nor accurate", noting that all card data can be compromised by visually examining the card and that even if an attack succeeded, card issuers offered fraud protection. Square later introduced strong encryption on its devices. In August 2012, Starbucks announced it would use Square to process transactions with customers who pay via debit or credit card. In December 2012, the service introduced virtual gift cards.

In May 2013, Block announced that its Square service was available in Japan after agreeing to a partnership with Sumitomo Mitsui Card Corporation. In May 2013, Square announced it would no longer support firearms-related transactions. In June 2013, the firm launched Square Market, which allows sellers to create a free online storefront with online payment processing functionality. Square Stand was introduced the same month.

In February 2014, Whole Foods Market announced it would use Square Register at select stores' sandwich counters, pizzerias and coffee, juice, wine and beer bars. In March 2014, the firm announced it would allow sellers to accept bitcoin on their own storefronts through Square Market. The seller takes no risk of bitcoin value fluctuations. In July 2014 the firm announced a card reader that would accept chip cards and contactless cards. Later in the year, Square added physical gift cards as a feature. Square Appointments, an online booking tool, was launched the same year. In December 2014, The Wall Street Journal and CNN reported that Starbucks had stopped accepting payments from Square Wallet, and that the partnership between Starbucks and Square had been a money loser for Square and was nearing an end.

In June 2015, Apple announced Square would release a reader device capable of accepting Apple Pay and other contactless payments. In the same year, Square launched a reader for Android and iOS that accepts contactless and chip card payments. Additionally, the firm launched Square Payroll for small business owners to process payroll.