Protein phosphatase‐1 (PP1) is a ubiquitous enzyme involved in multiple processes inside cells. PP1‐disrupting peptides (PDPs) are chemical tools that selectively bind to PP1 and release its activity. To restrict the activity of PDPs to a cellular compartment, we developed PDP‐Mem, a cell membrane‐targeting PDP. The membrane localization was achieved through the introduction of a palmitoylated lysine. PDP‐Mem was shown to activate PP1α in vitro and to localize to the membrane of HeLa Kyoto and U2OS cells. However, in cells, the combination of the polybasic sequence for cell penetration and the membrane targeting palmitoylated lysine activates the MAPK signaling pathway and induces cytoplasmic calcium release independently of PP1 activation. Therefore, when targeting peptides to cellular membranes, undesired effects induced by the targeting sequence and lipid modification need to be considered.