Sankt Marein bei Graz is a municipality in the district of Graz-Umgebung, in the Austrian state of Styria. It lies in the eastern part of the Graz-Umgebung district, roughly 20 kilometres east of the city of Graz, within the rolling hill country of eastern Styria.
The municipality is made up of several small villages and scattered settlements rather than a single dense town, a settlement pattern typical of the rural communities in this part of Austria. Its name links it to the nearby provincial capital, Graz, which serves as the region's main economic and administrative centre.
Like much of Styria, the area around Sankt Marein bei Graz is characterised by agriculture and open countryside, with farmland and wooded hills forming the surrounding landscape. The municipality belongs to the Graz-Umgebung district administration and is connected to Graz and neighbouring towns by road.