Stems erect, usually simple, 3–10 dm, usually sparsely pubescent, at least at nodes (or glabrous); rhizomes slender to somewhat thickened; bulblets absent. Leaves whorled; petiole absent or 0.1–0.3 cm, eciliate; blade elliptic to lanceolate or ovate, 3–12 × 0.8–3.5(–4.5) cm, base cuneate or rounded, slightly decurrent, margins entire, plane, glabrous or sometimes sparsely pubescent, apex acuminate or acute (rarely obtuse), surfaces densely to sparsely punctate, pubescent at least along abaxial veins and margins; venation pinnate-arcuate. Inflorescences axillary in leaves (distalmost axils sometimes without flowers), solitary flowers. Pedicels 1.5–3 cm, sparsely pubescent. Flowers: sepals 5, calyx streaked with dark resin canals, 3–6 mm, slightly stipitate-glandular, lobes lanceolate, margins thin; petals 5, corolla yellow with reddish base and, sometimes, margins, streaked with black or maroon resin canals, rotate, 5–8 mm, lobes with margins entire, apex acute to rounded, stipitate-glandular adaxially; filaments connate ca. 1.7 mm, shorter than corolla; staminodes absent. Capsules 3–3.5 mm, sometimes dark-punctate, glabrous. 2n = 84.
Phenology: Flowering summer.
Habitat: Dry to mesic hardwood forests, lowlands, fens, moist clearings, roadsides, and fields, rocky thickets and slopes, seashores
Elevation: 0-1000 m
N.B., Ont., Que., Ala., Conn., Del., D.C., Ga., Ill., Ind., Ky., Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Vt., Va., W.Va., Wis.
A hybrid (known only from one population in Washington County, North Carolina) of Lysimachia quadrifolia with L. loomisii has been called L. ×radfordii H. E. Ahles.