The district, 70 percent of which is made up of forests and mountains, comprises one-third of Nantou County and is bigger than the whole of Changhua County.
境內層巒疊嶂，全省最高峰玉山位於本鄉東南；山谷溪流交錯，陳有蘭溪與濁水溪分貫其間。信義鄉境內 70 % 為山地保留區及國有林地，適宜耕種的平原地有限。但也正因交通不便、開發不易，遂保持了原始山林之美。