Iroha Hill Road is a Japanese national highway (no. 120) which connects Nikkou's Umagae district to the banks of Lake Chuuzenji. The road, actually consisting of two one-way paths, is famous for its hairpin curves. Iroha Hill One, which goes from Lake Chuuzenji to Umagae, contains 28 curves, while Iroha Hill Two, going in the opposite direction, contains 20 curves. Iroha Hill One was established in 1954, Iroha Hill Two in 1965.

The name "Iroha" comes from the poem of the same name which uses each character of the Japanese hiragana exactly once; each of the 48 curves in the road is named after the character which it resembles.