Scientists had widely expected the milestone, after climate records were repeatedly broken. Since June, every month has been the world's hottest on record compared with the corresponding month in previous years.
"This has been a very exceptional year, climate-wise... in a league of its own, even when compared to other very warm years," C3S Director Carlo Buontempo said.
C3S confirmed 2023 as the hottest year in global temperature records going back to 1850. When checked against paleoclimatic data records from sources such as tree rings and air bubbles in glaciers, Buontempo said it was "very likely" the warmest year in the last 100,000 years.
On average, in 2023 the planet was 1.48 degrees Celsius warmer than in the 1850-1900 pre-industrial period, when humans began burning fossil fuels on an industrial scale, pumping carbon dioxide into the atmosphere.
Countries agreed in the 2015 Paris Agreement to try to prevent global warming surpassing 1.5C (2.7 degrees Fahrenheit), to avoid its most severe consequences.
The world has not breached that target - which refers to an average global temperature of 1.5C over decades - but C3S said that temperatures had exceeded the level on nearly half of the days of 2023 set "a dire precedent".
Professor of Climate Change at Newcastle University Hayley Fowler said the record-breaking year underlined the need to act "extremely urgently" to reduce emissions.
"The speed of change in the political world and the will to actually reduce greenhouse gas emissions is not matching the speed of change of extreme weather and warming," she said.
Despite the proliferation of governments' and companies' climate targets, CO2 emissions remain stubbornly high.
The world's CO2 emissions from burning coal, oil and gas hit record levels in 2023.
Last year, the concentration of CO2 in the atmosphere rose to the highest level recorded, of 419 parts per million, C3S said.
It was also the first year in which every day was more than 1C hotter than pre-industrial times. For the first time, two days - both in November - were 2C warmer than in the pre-industrial period.
Last year was 0.17C hotter than 2016, the previous hottest year - smashing the record by a "remarkable" margin, Buontempo said.
Alongside human-caused climate change, in 2023 temperatures were boosted by the El Nino weather phenomenon, which warms the surface waters in the eastern Pacific Ocean and contributes to higher global temperatures.
What scientists do not know yet is whether 2023's extreme heat is a sign that global warming is accelerating.
"Whether there's been a phase shift or a tipping point, or it's an anomalously warm year, we need more time and more scientific studies to understand," C3S Deputy Director Samantha Burgess said.
Each fraction of temperature increase exacerbates destructive weather disasters. In 2023, the hotter planet aggravated deadly heatwaves from China to Europe, extreme rain which caused floods killing thousands of people in Libya and Canada's worst wildfire season on record.
"Comparable small changes in global temperatures have huge impacts on people and ecosystems," Friederike Otto, a climate scientist who co-leads the World Weather Attribution global research collaboration, said.
"Every tenth of a degree matters," she added.
The economic consequences of climate change are also escalating. The U.S. suffered at least 25 climate and weather disasters with damages exceeding $1 billion, while droughts ravaged soybean crops in Argentina and wheat in Spain.
