Peercoin was the first cryptocurrency to implement a proof-of-stake consensus system.
Launched in 2012 by developers using the name Sunny King and with work by Scott Nadal, Peercoin introduced proof of stake as part of a hybrid security model. Its blockchain combined proof-of-work mining with proof-of-stake minting, allowing coin holders to help secure the network.
That hybrid design is an important detail. Peercoin was not purely proof of stake in the way many later networks were, so descriptions calling it the first cryptocurrency to use or implement proof of stake are more precise than calling it the first pure proof-of-stake blockchain.
Peercoin's approach influenced later projects such as Nxt and Blackcoin. Ethereum eventually moved from proof of work to proof of stake in 2022, while Cardano and Polkadot were designed much later around proof-of-stake-family systems.