Plant cytokinesis, which fundamentally differs from that in animals, involves de novo assembly of a plasma membrane precursor named the cell plate. How the transition from the cell plate to a plasma membrane occurs at the end of the plant cytokinesis remains poorly understood. Here, we describe with unprecedented spatiotemporal precision, the acquisition of plasma membrane identity upon cytokinesis through the lateral patterning of phosphatidylinositol 4,5-bisphosphate PI(4,5)P 2 at the newly formed cell plate membrane. We show that during late cytokinesis, opposing polarity domains are formed along the cell plate. Exclusion of PI(4,5)P 2 from the leading edge of the cell plate is controlled by SAC9, a putative phosphoinositide phosphatase. SAC9 colocalizes with MAP65-3, a key regulator of the cytokinesis, at the cell plate leading zone and regulates its function. In the sac9-3 mutant, the polar distribution of PI(4,5)P 2 at the cell plate is altered, leading to de-novo recruitment of the cytokinesis apparatus and to formation of an additional, ectopic cell plate insertion site. We proposed that PI(4,5)P 2 acts as a polar cue to spatially separate the expansion and maturation domains of the forming cell plate during the final steps of cytokinesis. One Sentence Summary The phosphoinositide PI(4,5)P 2 acts as an hallmark to guide the final step of plant cell division.