Phosphatidic acid (PA) is a common phospholipid that is a major constituent of cell membranes. Phosphatidic acid is the smallest of the phospholipids. Phosphatidic acid is not an abundant lipid constituent of any living organism, but it is extremely important as an intermediate in the biosynthesis of triacylglycerols and phospholipids. Indeed, it is often over-estimated in tissues as it can arise by inadvertent enzymatic hydrolysis during inappropriate storage or extraction conditions during analysis.

Phosphatidic acid is of considerable importance in cellular signalling in plants, for example in promoting pollen-tube growth, decreasing peroxide-induced cell death, and mediating the signalling processes that lead to responses to the plant hormone abscisic acid. Together these reactions constitute a signalling pathway that regulates water loss from plants.