Herbs annual or short-lived peren­nial, slender 2–7 dm, green, glabrous. Stems sprawl­ing or ascending, often branched from base, lax. Leaves mostly alter­nate, overlapping and smaller distally, subsessile; blade oblong at midstem to narrowly linear distally 8–38 × 1–11 mm, base obtuse to truncate. Inflorescences racemes. Flowers opposite to alternate, solitary in leaf axils, subsessile, tristylous; floral tube red-dotted on proximal half, narrowly obconic, 5–7 mm; epicalyx segments about equal to and more prominent than sepals; petals purple to rose, often with wihte base, obovate to oblong, 5–8 × 4–6 mm, stamens 12. Capsules septicidal or septi­fragal. Seeds many, oblong, 2 × 1 mm. 2n = 10.

Flowering summer–fall. Moist or wet places, seasonal pools, lake margins, springs; introduced; 0–500 m; Calif.; s Europe; sw Asia; n Africa; Atlantic Islands (Macaronesia); introduced also in South America (Brazil), c Europe, Pacific Islands (New Zealand), Australia.

Lythrum junceum was present in Alameda County in 1905. It is now locally abundant there and also established in Santa Clara County (B. Ertter & D. Gowen 2019). It was also collected in Massachusetts in 1883, but did not persist there.