亚甲蓝（Methylene blue），又称亚甲基蓝、次甲基蓝、次甲蓝、美蓝、品蓝、甲烯蓝、瑞士蓝（Swiss blue），国际非专利药品名称（INN）为methylthioninium chloride。是一种芳香杂环化合物。被用作化学指示剂、染料、生物染色剂和药物使用。亚甲蓝的水溶液在氧化性环境中蓝色，但遇锌、氨水等还原剂会被还原成无色形态。 亚甲蓝与弱酸性染料甲基蓝（Methyl blue）或pH指示剂甲基紫（methyl violet，又称「龙胆紫」）不同，应注意勿混用。

Methylene blue (CI 52015), also known as methylthioninium chloride, has many uses in biology and chemistry; for example, it can be used as a stain and as a medication.

It is a heterocyclic aromatic chemical compound with the chemical formula C₁₆H₁₈N₃SCl. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in water. The hydrated form has 3 molecules of water per unit of methylene blue. Methylene blue should not be confused with methyl blue, another histology stain, new methylene blue, nor with the methyl violets often used as pH indicators.