Ethereum Classic is a blockchain-based distributed computing platform that offers smart contract (scripting) functionality. Ethereum Classic was created in a hard fork with the mainline Ethereum blockchain, and maintains the original, unaltered ledger prior to the attempt to reverse a hacking attack on the Ethereum-based DAO in July 2016. It is now the largest smart contract platform secured by a proof-of-work consensus mechanism, following Ethereum's transition to proof-of-stake in 2022.

It is open source and supports a modified version of Nakamoto consensus via transaction-based state transitions executed on a public Ethereum Virtual Machine (EVM).

Ethereum Classic maintains the original, unaltered history of the Ethereum network. The Ethereum project's mainnet was initially released via Frontier on 30 July 2015. However, due to a hack of a third-party project, The DAO, the Ethereum Foundation created a new version of the Ethereum mainnet on 20 July 2016 with an irregular state change implemented that erased the DAO theft from the Ethereum blockchain history. The Ethereum Foundation applied their trademark to the new, altered version of the Ethereum blockchain.

The older, unaltered version of Ethereum was renamed and continued on as Ethereum Classic.

Ethereum Classic's native Ether token is a cryptocurrency traded on digital currency exchanges under the currency code ETC. Ether is created as a reward to network nodes for a process known as "mining", which validates computations performed on Ethereum Classic's EVM.

Several codenamed prototypes of the Ethereum platform were developed by the Ethereum Foundation, as part of their proof-of-concept series, prior to the official launch of the Frontier network. Ethereum Classic followed this codebase after the DAO incident.

On 20 July 2016, as a result of the exploitation of a flaw in The DAO project's smart contract software, and subsequent theft of $50 million worth of Ether, the Ethereum network split into two separate blockchains – the altered history was named Ethereum (ETH) and the unaltered history was named Ethereum Classic (ETC).

On 28 May 2016, a paper was released detailing security vulnerabilities with the DAO that could allow Ether to be stolen. On 9 June 2016, Peter Vessenes publicly disclosed the existence of a critical security vulnerability overlooked in many Solidity contracts, a recursive call bug. On 12 June 2016, Stephan Tual publicly claimed that the DAO funds were safe despite the newly-discovered critical security flaw.

On 15 July 2016, a short notice on-chain vote was held on the DAO hard fork. Of the 82,054,716 ETH in existence, only 4,542,416 voted, for a total voter turn out of 5.5% of the total supply on 16 July 2016; 3,964,516 ETH (87%) voted in favor, 1/4 of which came from a single address, and 577,899 ETH (13%) opposed the DAO fork. The expedited process of the carbon vote drew criticism from opponents of the DAO fork. Proponents of the fork were quick to market the vote as an effective consensus mechanism, pushing forward with the DAO fork four days later.