Vocabulary What does 海王星 (かいおうせい, kaiousei) mean? 海王星 (かいおうせい, kaiousei) means Neptune (planet). Quick factsReadingかいおうせいRomajikaiouseiPart of speechnoun 海王星かいおうせいkaiouseiNeptune (planet)1noun Neptune (planet)Kanji in this word海sea王king星star