Ethereum transitioned to proof-of-stake in 2022 through an upgrade known as The Merge.
The mainnet transition occurred on 15 September 2022. Ethereum’s original execution layer joined the Beacon Chain, a separately operating proof-of-stake chain, so the network could replace proof-of-work mining with validator-based block production.
The Beacon Chain had launched in December 2020, allowing the proof-of-stake system to operate and accumulate testing experience before it became Ethereum’s main consensus mechanism. The Merge itself was associated especially with the Bellatrix and Paris upgrades.
A common misconception is that The Merge made Ethereum immediately faster or cheaper. Its central achievement was changing how the network reaches consensus, while later upgrades addressed other scaling and usability goals. The transition also reduced Ethereum’s energy consumption by more than 99 percent.