Here, we describe an unusual new species, Lysimachia brevianthera X.W. Li, in Lysimachia subgen. Idiophyton from China. It has the filaments longer than the basifixed anthers, at the base fused into a narrow ring connate with the corolla; leaves congested at the upper part of the stem; and grayish black capsules. Lysimachia brevianthera is similar to L. laxa in having a winged stem and the free part of filaments longer than 2 mm, but differs from it by the unbranched stems (vs. many branched stems), larger flowers and grayish black capsules (vs. light brown capsules). A phylogenetic analysis supports the placement of L. brevianthera in subgen. Idiophyton.