Smart contracts are key software components of blockchain applications. Recently, a great number of bugs and security vulnerabilities have been exposed in smart contracts on the Ethereum blockchain. This resulted in extensive research efforts in the formal verification of smart contracts at the international level. To obtain highly trustworthy verification results, the formalization of programming languages for smart contracts is necessary. This work formalizes Yul-the Ethereum intermediate language. The core of this formalization consists of the first formal definitions of the type system and the small-step operational semantics for Yul. The semantics is executable, and it is validated using a test suite of 120 Yul programs. The proposed formalization is performed in the Isabelle/HOL proof assistant. It lays a solid foundation for the formal verification of the correctness and security of Ethereum smart contracts through theorem proving.