Honshu (本州, Honshū, pronounced [hoꜜɰ̃ɕɯː] (listen); lit. "main province"), historically called Hondo (本土, lit. "mainland"), is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island separates the Sea of Japan, which lies to its north and west, from the North Pacific Ocean to the south and east. It is the seventh-largest island in the world, and the second-most populous after the Indonesian island of Java.

Honshu had a population of 104 million as of 2017, constituting 81.3% of the entire population of Japan, and is mostly concentrated in the coastal areas and plains. Approximately 30% of the total population resides in the Greater Tokyo Area on the Kantō Plain. As the historical center of Japanese cultural and political power, the island includes several past Japanese capitals, including Kyōto, Nara and Kamakura. Much of the island's southern shore forms part of the Taiheiyō Belt, a megalopolis that spans several of the Japanese islands. Honshu contains Japan's highest mountain, Mount Fuji, and its largest lake, Lake Biwa.

Most of Japan's industry is located in a belt running along Honshu's southern coast, from Tokyo to Nagoya, Kyōto, Osaka, Kobe, and Hiroshima; by contrast, the economy along the northwestern Sea of Japan coast is largely based on fishing and agriculture. The island is linked to the other three major Japanese islands by a number of bridges and tunnels. The island primarily shares two climates, with Northern Honshu being mainly humid continental climate while the south has a humid subtropical climate.