四甲基氢氧化铵（TMAH或TMAOH）是一种分子式为 N(CH₃)₄OH 的季铵碱，也是这类化合物中最简单的一种。这种物质只在五水合物时是一种相对稳定的固体形态。商业上，TMAH通常以浓的水或甲醇溶液的形式销售。其固体和溶液均为无色，不纯时为黄色。纯净的TMAH几乎没有气味，但样品常有强烈的鱼腥味，因其含有三甲胺的不纯物。TMAH有大量、广泛的工业和科研用途，例如在黄光制程中当显影液(去光阻液)。

Tetramethylammonium hydroxide (TMAH or TMAOH) is a quaternary ammonium salt with the molecular formula N(CH₃)₄OH, and is one of the simplest members of this class of organic compounds. This substance is known in a (relatively) stable solid form only as the pentahydrate. Commercially, the forms in which TMAH is most commonly encountered are as concentrated solutions in water or methanol. The solid and solutions are colorless, or yellowish if impure. Although TMAH has virtually no odor when pure, samples often have a strongly fishy smell from the trimethylamine which is commonly present as an impurity. TMAH has numerous and diverse industrial and research applications (see below).