Large amounts of CO2 are transferred from Earth's interior to the surface by volcanism. On a geological time scale, the rate of CO2 emission has controlled the evolution of Earth's atmosphere and climate, as well as the dynamic processes that take place in the mantle and core. The total rate of natural CO2 emission from Earth has been estimated on the basis of CO2 flux from arc, mid-ocean-ridge, and hotspot volcanism. However, previous estimates have overlooked the CO2 emitted from a recently discovered type of volcanism-petit-spot volcanism-that occurs on the deep-sea floor. Here, we measure the CO2 and H2O contents of glassy basalts produced by petit-spot volcanism and estimate the initial contents to be >5 wt% and 1.0-1.1 wt%, respectively. Based on these values and magma flux of petitspot volcanism, we show that the rate of CO2 emission from petitspot volcanoes (2.7-5.4 × 1011 g CO2 yr-1) is a few percent of the CO2 emissions from arc and mid-ocean-ridge volcanism, and up to ~14% of that from hotspot volcanism. Thus, the contribution to the carbon cycle on Earth of the large amounts of CO2 that have been emitted from the deep-sea floor by petit-spot volcanism has not previously been recognized.