Deloitte Touche Tohmatsu Limited (), commonly referred to as Deloitte, is an international professional services network headquartered in London, England. Deloitte is the largest professional services network by revenue and number of professionals in the world and is considered one of the Big Four accounting firms along with EY (Ernst & Young), KPMG and PricewaterhouseCoopers (PWC).

The firm was founded by William Welch Deloitte in London in 1845 and expanded into the United States in 1890. It merged with Haskins & Sells to form Deloitte Haskins & Sells in 1972 and with Touche Ross in the US to form Deloitte & Touche in 1989. In 1993, the international firm was renamed Deloitte Touche Tohmatsu, later abbreviated to Deloitte. In 2002, Arthur Andersen's practice in the UK as well as several of that firm's practices in Europe and North and South America agreed to merge with Deloitte. Subsequent acquisitions have included Monitor Group, a large strategy consulting business, in January 2013. The international firm is a UK private company, limited by guarantee, supported by a network of independent legal entities.

Deloitte provides audit, consulting, financial advisory, risk advisory, tax, and legal services with approximately 415,000 professionals globally. In FY 2021, the network earned revenues of US$50.2 billion in aggregate. As of 2020, Deloitte is the third-largest privately-owned company in the United States, according to Forbes. The firm has sponsored a number of activities and events including the 2012 Summer Olympics.

Controversies involving the firm, in addition to litigation surrounding a few of its audits, have included its involvement in a "potentially misleading" report on illicit tobacco trading in Australia, the fact that it suffered a major cyber-attack which breached client confidentiality as well as exposing extensive employee information in September 2017, its role as internal auditor of the insolvent contractor Carillion and its role as external auditor of Autonomy which was accused of "accounting improprieties" that contributed to an £8.8 billion write-down of value following its acquisition by Hewlett-Packard in 2011.