Silene alexandri is a subshrub that grows to 30–60 cm tall. The stems are erect, essentially glabrous, and often swollen at the nodes. The leaves are simple, opposite, or whorled, with elliptic-oblanceolate blades measuring 30–65 mm long and 6–14 mm wide. The leaf apex is acuminate, and the base is often connate around the stem, gradually tapering to the base. The leaf surfaces are glabrous, with entire margins that are sparsely ciliate. Stipules are absent. The flowers are arranged in open, paniculate cymes with glabrous pedicels 10–19 mm long. The flowers are bisexual (perfect) and actinomorphic. The calyx is tubular, 5-lobed, measuring 19–25 mm long, with 10 nerves. It is glabrous, with rounded lobes, notched apex, and scarious, sparsely ciliate margins. The corolla consists of 5 white petals, with blades 8–10 mm long and cleft, extending 4–6 mm beyond the calyx. The stamens are exserted beyond the calyx. The ovary is superior, 1-celled or incompletely 2–4 celled, on a well-developed stipe that is adnate to the staminal filament bases and petal bases, referred to as a carpophore. The styles are 3, and the carpophore measures 13–17 mm long. The fruit is an oblong, ovoid capsule, 3-celled in the lower part, measuring about 14–16 mm long. It dehisces by six or rarely three apical teeth or valves. The carpophore is 13–17 mm long. It is known only from remnant dry forests and shrubland between 610 – 760 meters.