维生素D缺乏症是缺乏维生素D所表现出来的一系列病症。这种疾病可能是由于人体摄入的维生素D不足，并且没有接受足够多的阳光照射（准确地说是阳光中的中波紫外线B光）导致的，也有可能是体内发生的紊乱导致维生素D的吸收受限导致维生素D缺乏症，还有可能是因为肝脏、肾脏或者遗传因素导致的紊乱使得维生素D转换成为具有活性的代谢产物的过程受损。

维生素D缺乏症可以引发骨矿化受损以及多种与骨骼相关的疾病，儿童可能会出现佝偻病，成人可能会出现骨软化和骨质疏松症。

Hypovitaminosis D is a deficiency of vitamin D. It can result from inadequate nutritional intake of vitamin D and/or inadequate sunlight exposure (in particular sunlight with adequate ultraviolet B rays), disorders limiting vitamin D absorption, and conditions impairing vitamin D conversion into active metabolites—including certain liver, kidney, and hereditary disorders. Deficiency impairs bone mineralization, leading to bone softening diseases as rickets in children and osteomalacia and osteoporosis in adults. Emerging evidence suggests vitamin D plays a role in the pathogenesis of non-alcoholic fatty liver disease (NAFLD).