The average global temperature reached 17.01C (62.62F) on Monday, according to the US National Centres for Environmental Prediction.
The figure surpasses the previous record of 16.92C (62.46F) – set back in August 2016.
It comes as the southern US and China have been hit by heatwaves, while temperatures in North Africa have neared 50C (122F).
Experts have blamed a combination of climate change and an emerging El Nino weather pattern.
“This is not a milestone we should be celebrating,” said climate scientist Friederike Otto of the Grantham Institute for Climate Change and the Environment at Britain’s Imperial College London.
“It’s a death sentence for people and ecosystems.”
Earlier this year, the United Nations warned of higher global temperatures and new heat records due to climate change and the return of El Nino.
El Nino is a rotating weather pattern associated with warmer than normal ocean surface temperatures in the central and eastern tropical Pacific and rainier, cooler conditions in the south and warmer conditions in parts of the north.
For three years, the opposite of El Nino – the cooling La Nina weather pattern – has been dominant in the Pacific Ocean.
This has lowered global temperatures slightly – but 2023 has seen the return of the warmer counterpart.
The southern US has been suffering under an intense heat dome in recent weeks, while in China, an enduring heatwave has seen temperatures rise above 35C (95F).
Even Antarctica, currently in its winter, has registered anomalously high temperatures.
In the UK, the Met Office recorded June as the warmest since records began in 1884, with a mean average temperature of 15.8C.
The temperature – an average daytime and nighttime temperature from across the UK in June – surpassed the previous record of 14.9C, set in both 1940 and 1976.
Zeke Hausfather, a research scientist at Berkeley Earth, warned more records could be broken this year, due to an increase in emissions and El Nino.