Osogovo (Bulgarian and Macedonian Cyrillic: ) or Osogovska Planina (Осоговска планина or Осоговски Планини) is a mountain and ski resort between the southwestern part of Bulgaria (Kyustendil Province) and the northeastern part of North Macedonia (Kočani and Kriva Palanka municipalities). It is about long and wide, the highest peak being Ruen (Руен) at 2251 m, which constitutes the main orthographic knot on the very border between Bulgaria and North Macedonia. The steepest slope of the mountain is in the west, stretching down from Carev Vrv (Царев Врв; Tsar's Peak).