Terrestrial planets covered globally with thick oceans (termed ocean planets) in the habitable zone were previously inferred to have extremely hot climates in most cases. This is because H 2 O high-pressure (HP) ice on the seafloor prevents chemical weathering and, thus, removal of atmospheric CO 2. Previous studies, however, ignored melting of the HP ice and horizontal variation in heat flux from oceanic crusts. Here we examine whether high heat fluxes near the mid-ocean ridge melt the HP ice and thereby remove atmospheric CO 2. We develop integrated climate models of an Earth-size ocean planet with plate tectonics for different ocean masses, which include the effects of HP ice melting, seafloor weathering, and the carbonate-silicate geochemical carbon cycle. We find that the heat flux near the mid-ocean ridge is high enough to melt the ice, enabling seafloor weathering. In contrast to the previous theoretical prediction, we show that climates of terrestrial planets with massive oceans lapse into extremely cold ones (or snowball states) with CO 2-poor atmospheres. Such extremely cold climates are achieved mainly because the HP ice melting fixes seafloor temperature at the melting temperature, thereby keeping a high weathering flux regardless of surface temperature. We estimate that ocean planets with oceans several tens of the Earth's ocean mass no longer maintain temperate climates. These results suggest that terrestrial planets with extremely cold climates exist even in the habitable zone beyond the solar system, given the frequency of water-rich planets predicted by planet formation theories.Read less <