What is the name of the 31-syllable Japanese poem?

The name of the traditional 31-unit Japanese poem is tanka.

Tanka is a short Japanese poetic form traditionally arranged in five phrases with a 5–7–5–7–7 pattern. English descriptions often call it a 31-syllable poem, but Japanese verse counts morae, or sound units, rather than syllables in the English sense. That distinction is why a literal 31-syllable translation can be misleading.

The form has a long history. Tanka developed from earlier Japanese verse and became a major vehicle for personal expression, especially in poems about love, nature, longing, travel, and court life. The ancient anthology Man’yōshū, compiled in the eighth century, preserves many early examples.

Tanka is related to haiku because its opening 5–7–5 section resembles the structure that later became the independent haiku tradition. The final 7–7 phrases give tanka more room for reflection or emotional development. Senryū, by contrast, is a related short form focused more on human behavior and social satire.