Phosphatidylinositol 4-phosphate (PI4P) is an anionic phospholipid, which has been described as a master regulator of the Golgi apparatus in eukaryotic cells. However, recent evidence suggests that PI4P mainly accumulates at the plasma membrane in all plant cells analyzed so far. Besides, many functions that are typically attributed to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] in animal and yeast cells are also supported by PI4P in plants. For example, PI4P is the key anionic lipid that powers the strong electrostatic properties of the plasma membrane. PI4P is also required for the establishment of stable membrane contacts between the endoplasmic reticulum (ER) and the plasma membrane, for exocytosis and to support signaling pathways. Thus, we propose that PI4P has a prominent role in specifying the identity of the plasma membrane and in supporting some of its key functions and should be considered a hallmark lipid of this compartment.