白质（英语：White matter，拉丁语：substantia alba）是中枢神经系统中主要的三个组成元素之一，与灰质、黑质并列。大脑剖面中的白质组织是由大量的髓磷脂（脂质）组成，在裸视观察下呈现白色。 白质由被髓鞘包覆着的神经轴突组成，控制着神经元共享的信号，协调脑区之间的正常运作。人类到了约二十岁时，白质才会在不同脑区逐渐发育完全，而其生长的时机与成熟程度，会影响到学习、自我控制与精神疾病，例如精神分裂、自闭症与病态性说谎，青少年的「年少轻狂」的原因之一也是由于白质未发育完全。

White matter, named for its relatively light appearance resulting from the lipid content of myelin, refers to axon tracts and commissures.

White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color in prepared specimens is due to its usual preservation in formaldehyde.