始新世（英语：Eocene）是地质时代中古近纪（Paleogene）的第二个主要分期，大约开始于5580万年前-终于3400万年前，介于古新世（Paleocene）与渐新世（Oligocene）之间。始新世（Eocene）之名，字源来自希腊文eos（开端）和ceno （新），指的是现代哺乳动物群出现的开始。始新世常被划分成早期（距今5580万年∼5,200万年）、中期（距今5,200万∼4,360万年）和晚期（距今4,360万∼3400万年）。始新世早期最令人受到注意的是，原始的现代哺乳动物的出现。始新世结束于一个被称作大置换（物种连续哺乳动物性上的“大断裂”）的大型生物集群灭绝。此一事件可能是关于一或数颗的大火流星撞击西伯利亚以及现在的切萨皮克湾所造成。

The Eocene (pronunciation: /ˈiːəˌsiːn, ˈiːoʊ-/; symbol Eo) Epoch, lasting from 56 to 33.9 million years ago, is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope C in the atmosphere was exceptionally low in comparison with the more common isotope C. The end is set at a major extinction event called the Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event, which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. As with other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain.