OneCoin is a fraudulent cryptocurrency scheme conducted by offshore companies OneCoin Ltd (based in Bulgaria and registered in UAE) and OneLife Network Ltd (registered in Belize), both founded by Ruja Ignatova in concert with Sebastian Greenwood. OneCoin is considered a Ponzi scheme due to its organisational structure of paying early investors using money obtained from newer ones. It was also a pyramid scheme due to the recruiting of investors without providing any actual product.

The company maintained its own database of coins rather than using a blockchain and had no mining process which limited its ability to release and circulate coins. Many of the people central to OneCoin had been previously involved in similar schemes and business malpractice. OneCoin was described by The Times as "one of the biggest scams in history".

US prosecutors alleged the scheme brought in approximately $4 billion worldwide. In China, law enforcement recovered 1.7 billion yuan (US$267.5 million) while prosecuting 98 people. Ruja Ignatova disappeared in 2017 when a secret US warrant was filed for her arrest, and her brother, Konstantin Ignatov, took her position at the company. Most of the leaders of the company disappeared or were arrested, though Ruja Ignatova has escaped arrest. Greenwood was arrested in 2018, as was Konstantin Ignatov in March 2019.

In November 2019, Konstantin Ignatov pleaded guilty to charges of money laundering and fraud. The total maximum sentence for the charges is 90 years in prison. In 2022 Greenwood pleaded guilty, and in 2023 was sentenced to 20 years for wire fraud and money laundering.

OneCoin was launched in late 2014. It was not a decentralized cryptocurrency but rather a centralized pyramid scheme hosted on OneCoin Ltd's servers which was marketed as a cryptocurrency. The scheme was set up by Ruja Ignatova with Sebastian Greenwood acting as the lead distributor in the pyramid.

According to OneCoin, its main business was selling educational material for cryptocurrency trading: investors could buy "educational packages" costing anywhere from 100 euros to 118,000 euros, or—according to one industry blog—225,500 euros. According to a suit filed by former investors, much of the content in those packages was plagiarized from various free sources, including Wikipedia.^([better source needed]) Investors also received "tokens" that could supposedly be assigned to mine OneCoins, although no actual mining process existed.

Mining was said to be taking place at two sites in Bulgaria and one in Hong Kong.

In typical OneCoin recruiting meetings, recruiters would focus on cryptocurrency investment, and the "educational material" was barely mentioned.^([better source needed])

On 30 September 2015, Bulgaria's Financial Supervision Commission (FSC) issued a warning of potential risks in new cryptocurrencies, citing OneCoin as an example. After the warning, OneCoin ceased all activity in Bulgaria and started to use banks in foreign countries to handle wire transfers from participants.

In February 2016, the British newspaper the Daily Mirror wrote that OneCoin / OneLife is a get-rich-quick scheme scam and a cult, calling it "virtually worthless".

The company and the scheme was on the observation lists of many authorities; among them are authorities in Bulgaria, Finland, Sweden, Norway and Latvia. Authorities in many countries warned of potential risks involved in businesses like OneCoin and undertook prosecutions against persons linked to OneCoin, including CEO Ruja Ignatova and her brother Konstantin Ignatov.

In March 2016, the Direct Selling Association in Norway warned against OneCoin, comparing it to a pyramid scheme.

In March 2017, the Croatian National Bank (HNB) advised the public to "exercise a high degree of caution" in decisions involving OneCoin, noted that OneCoin operations are not supervised by the HNB, and warned that possible losses will be fully borne by the investors.

On 23 April 2017, Indian police arrested 18 people in Navi Mumbai for organizing a OneCoin recruitment event. The police attended the event undercover to judge the accusations before they decided to act. Further investigation was begun with the intent of revealing the higher levels of the pyramid. In May, the investigation recovered Rs 24.57 crores ($3.32 million USD) in nine bank accounts. A further Rs 75 crores ($10.12 million USD) were transferred out before authorities were able to seize it.

Early in the month of May, two more people were arrested and a further Rs 24 crores ($3.24 million USD) were seized from bank accounts. A special investigation team of 15 individuals including 4 assistant police inspectors was formed under Senior Police Inspector Shivaji Awate with the stated goal of following the money trail and possibly lead to further arrests.

On 27 April 2017, Germany's Federal Financial Supervisory Authority (BaFin) issued cease and desist orders to Onecoin Ltd, Dubai, and OneLife Network Ltd, Belize. The Authority concluded that trading in OneCoins was fraudulent "own funds" trading.