Marajó, island, c.150 mi (240 km) long and c.100 mi (160 km) wide, N Brazil, at the mouth of the Amazon River. It divides the river into the Amazon proper and the Pará. Cattle are raised on the extensive eastern grasslands, and water buffaloes are bred in the low, swampy west. The island is famous for its prehistoric mounds, which yield handsome pottery.