Arctic Ocean is considered especially vulnerable to ocean acidification. Here we summarize current understandings of reasons of the vulnerability, state and future perspective of the ocean acidification in the Arctic Ocean, with an emphasis on regional variability. In general, Arctic Ocean has low calcium carbonate saturation state (Ω) and low pH buffer capacity to added carbon dioxide, because of its low temperature and dilution by various freshwater sources. In coastal shelf area, local physical and biogeochemical processes, such as river discharge and high biological activity, characterize Ω and pH in each region. Recent climate change is affecting each of these processes, to complicate ocean acidification in the Arctic Ocean. Despite the increasing attention, long-term observations are still insufficient for most part of the Arctic Ocean. Understanding and monitoring of Arctic Ocean would provide much knowledge about biogeochemical consequences of ocean acidification and concurrent climate change, in order to better predict future of our ocean.