Unterfladnitz is a former municipality in the district of Weiz, in the Austrian state of Styria. It lies in the eastern part of Styria, within the Weiz district, a region of rolling hills and farmland in the foothills of the eastern Alps.
Since the 2015 Styria municipal structural reform, Unterfladnitz is no longer an independent municipality and has been part of the municipality of Sankt Ruprecht an der Raab. The reform merged a number of small Styrian municipalities into larger administrative units, and Unterfladnitz was among those absorbed.
Its administrative history is therefore its most notable characteristic: it exists today as a locality within Sankt Ruprecht an der Raab rather than as a municipality in its own right. The village lies in the Raab valley area, near Sankt Ruprecht an der Raab, in the Weiz district of Styria.