Semseyite is a rarely occurring sulfosalt mineral and is part of the class of lead antimony sulfides.

The mineral forms dark gray to black aggregates.

Semseyite was first described for an occurrence in the Felsöbánya mine in Baia Sprie, Romania in 1881 by József Sándor Krenner (1839–1920).

The mineral was named after Hungarian mineralogist Andor von Semsey (1833–1923).

[6] It occurs in association with bournonite, jamesonite, sphalerite, zinkenite, sorbyite, guettardite, jordanite, diaphorite, galena, pyrite, chalcopyrite, tetrahedrite, arsenopyrite and siderite.