The Holocene Climate Optimum (HCO) was a warm period that occurred in the interval roughly 9,500 to 5,500 years ago BP, with a thermal maximum around 8000 years BP. It has also been known by many other names, such as Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, Hypsithermal, and Mid-Holocene Warm Period.

The warm period was followed by a gradual decline, of about 0.1 to 0.3 °C per millennium, until about two centuries ago (when this trend was rapidly reversed due to human-produced greenhouse gas emissions). However, on a sub-millennial scale, there were regional warm periods superimposed on this decline.

• For other temperature fluctuations, see temperature record.
• For other past climate fluctuation, see paleoclimatology.
• For the pollen zone and Blytt–Sernander period, associated with the climate optimum, see Atlantic (period).