Indoor plants are often promoted as a natural means to regulate indoor air quality and manage humidity through evapotranspiration. However, their capacity to reduce excessive indoor moisture remains uncertain. This systematic review investigates whether indoor plants effectively lower relative humidity (RH) in building environments or, instead, contribute to increased moisture levels. A comprehensive literature search was conducted following PRISMA guidelines across Web of Science, Scopus, and PubMed. Of 5,948 initial records, 37 empirical, English-language studies met the inclusion criteria and were analysed. These studies, published between 2004 and 2025, encompassed a variety of settings including sealed chambers, office spaces, classrooms, and households and explored a wide range of species, experimental setups, and measurement tools. The findings reveal a consistent trend: Indoor plants primarily act as sources of humidity. Most studies reported RH increases between 2% and over 40%, with some chamber studies reaching 100% RH. In room-scale environments, RH was up to 65% in planted settings compared to 15–55% in control spaces. Only four studies reported minor RH reductions, but these were context-specific and lacked evidence of net atmospheric moisture absorption. None of the studies demonstrated that plants reduced RH beyond the moisture they released through evapotranspiration. This review offers new evidence that contradicts popular claims about the dehumidifying capabilities of indoor plants. While they may enhance comfort in dry conditions, they do not provide a reliable method for reducing excess indoor humidity. These findings highlight the need for standardized research protocols and further investigation into plant-environment interactions to inform indoor air quality strategies.