The new system will use 99.95 percent less energy, according to the Ethereum Foundation.
Leading cryptocurrency figures have hailed the completion of one of the biggest software upgrades the sector has ever attempted, an overhaul of the Ethereum blockchain aimed at reducing its massive energy consumption.
"And we finalized!" tweeted Ethereum's co-creator Vitalik Buterin on Thursday, calling it a "big moment for the Ethereum ecosystem".
Buterin quoted research claiming that the "merge", as developers have called the software upgrade, would reduce global electricity consumption by 0.2 percent.
Enthusiasts hope a more energy-efficient Ethereum will spur wider adoption of blockchain technology, particularly for banks and financial firms to automate backend processes.
But so far the technology has been used largely to create speculative financial products.
And critics remain sceptical of the energy-saving claims, pointing out that it is unclear how much energy the new system will need.
Blockchain company Consensys called it a "monumental technological milestone" but the scale of the work and potential for glitches led several companies and major exchanges to halt trading during the merge process.
The biggest exchange, Binance, said on Thursday it had resumed trading in ether, the native currency of Ethereum, tweeting: "The Ethereum Merge is complete."
Ether was down slightly in early trading and has lost more than half of its value since the start of the year, suffering a rout along with the rest of the crypto world as investors shied away from risky assets.
Ether accounts for almost 20 percent of a cryptocurrency market valued at around $1 trillion, according to the site CoinGecko.
READ MORE: What is ‘the Merge’ and how will it change Ethereum?
And we finalized!— vitalik.eth (@VitalikButerin) September 15, 2022
Happy merge all. This is a big moment for the Ethereum ecosystem. Everyone who helped make the merge happen should feel very proud today.
'Proof of stake'
The upgrade changes the way transactions are logged on the Ethereum blockchain.
From the start of Ethereum in 2015, so-called crypto miners competed for the prize of adding entries to the blockchain.
They used vast computer power to solve complex equations with only the winners getting the reward, a mechanism known as "proof of work".
Developers had spent years working on an energy-efficient version of Ethereum, a digital ledger that underpins tens of billions of dollars worth of cryptocurrencies, digital tokens (NFTs), games and apps.
Ethereum is the second most important blockchain after bitcoin, but it has faced criticism for burning through more power each year than New Zealand.
The new system scraps the competition element and eliminates the miners and their energy-guzzling computer stacks.
Instead, potential "validators" need to put up 32 ether (worth $55,000) with the winner chosen in a lottery-style system to update the chain and receive the reward, a system known as "proof of stake".
READ MORE: Why have crypto markets collapsed?