Castroserna de Abajo is a municipality in the province of Segovia, in the autonomous community of Castile and León, Spain. It is a small rural settlement in this landlocked region of the country's interior.
The municipality is sparsely populated. According to the 2004 census carried out by Spain's National Statistics Institute (INE), Castroserna de Abajo had a population of 49 inhabitants, reflecting the low population density typical of many villages in the surrounding area.