Signet Jewelers Ltd. (Ratner Group 1949-1993 then Signet Group plc to September 2008) is the world's largest retailer of diamond jewelry. The company is domiciled in Bermuda and headquartered in Akron, Ohio, and is listed on the New York Stock Exchange. As of 14 March 2016 the company has been delisted from the London Stock Exchange, following reports that less than 1% of their annual trading volume was conducted via the platform. The group operates in the middle market jewelry segment and has number one positions in the U.S., Canada and UK speciality jewelry markets. Certain brands (Jared in the U.S. and H. Samuel/Ernest Jones/Leslie Davis in the UK) operate in the upper middle market. Signet Jewelers owns and operates the companies Zales, Kay Jewelers, and Jared.

The group was founded in 1949 and grew organically before expanding rapidly through a series of acquisitions in the late 1980s and early 1990s, and was formerly known as the Ratner Group.

Gerald Ratner who built it from 130 stores to 2500, a previous CEO, made possibly the most famous gaffe in twentieth century British business when he explained to a major business conference that the reason why one of his products was so cheap was that it was "total crap". He then went on to unfavourably compare some of the company's earrings with a 99p prawn sandwich. His remarks were gleefully reported by the media. The company lost over 500 million pounds off its share price and consumers subsequently avoided the Ratner branded stores, nearly 300 of which were closed between January 1992 and May 1994 as the group went through a financial restructuring. Ratner resigned in November 1992 and the group changed its name to Signet Group plc in September 1993.

This perceived lack of judgement and contempt for the customer gave rise to the expression "doing a Ratner".

The company moved its primary stock market listing from the London to the New York Stock Exchange on September 11, 2008, changing its name to Signet Jewelers Limited in the process. The firm also moved its country of domicile from the United Kingdom to Bermuda on the same day, although it retains headquarters in Akron, OH.

In February 2014, Signet Jewelers Ltd. agreed to buy Zale Corporation, with Zale shareholders receiving USD$21 a share in cash in USD$1.4 billion deal. This merger has created a $6.2 billion firm.

In July 2017, Virginia Drosos was appointed as the new CEO of Signet Jewelers Ltd., replacing Mark Light who had served as CEO since October 2014.

In August 2017, it was announced that Signet Jewelers Ltd. agreed to buy R2Net, owner of online jewelry retailer James Allen, for $328 million.

Signet operates approximately 5,000 stores.

In May 2017, one of Signet's subsidiaries, Sterling Jewelers, settled a federal civil lawsuit brought by the U.S. Equal Employment Opportunity Commission accusing it of discriminating against female employees. Signet is also subject to at least two class actions through Sterling and one of its subsidiary, Jared The Galleria of Jewelry. It is being sued by 44,000 female employees and former employees for discrimination. The action was launched in 2008 and Sterling was still fighting it in 2017; it is set to go to trial in 2018.