Sign-In with Ethereum is a new form of authentication that enables users to control their digital identity with their Ethereum account and ENS profile instead of relying on a traditional intermediary. Already used throughout web3, this is an effort to standardize the method with best practices and to make it easier for web2 services to adopt it. Sign-In with Ethereum describes how Ethereum accounts authenticate with off-chain services by signing a standard message format parameterized by scope, session details, and security mechanisms (e.g., a nonce). The goals of this specification are to provide a self-custodied alternative to centralized identity providers, improve interoperability across off-chain services for Ethereum-based authentication, and provide wallet vendors a consistent machine-readable message format to achieve improved user experiences and consent management.