The receptor tyrosine kinase EphA2 binds to its ephrinA1 ligand, which is found at the membrane of opposing cells. EphA2 thus binds to ephrinA1 at cell-to-cell contacts, and it causes cell repulsion through activation of intracellular signaling pathways that control cytoskeletal dynamics. As a result, this signaling axis controls contact-dependent cell communication that drives cell adhesion, migration, morphology, and survival. Misregulation of the EphA2-ephrinA1 signaling axis causes aberrant cell-to-cell contacts that contribute to metastasis. Solid tumors are characterized by an acidic extracellular medium. We took advantage of this tumor property to create TYPE7, a transmembrane ligand for EphA2. TYPE7 is a peptide obtained by adding glutamic acid residues to the sequence of the transmembrane domain of EphA2. TYPE7 is highly soluble and interacts with the surface of lipid membranes at neutral pH, while acidity triggers protonation of the glutamic acids residues, which causes transmembrane insertion. TYPE7 works as a conditional transmembrane domain, where membrane insertion is triggered by a pH drop. TYPE7 strongly binds to endogenous EphA2 in cells. Interestingly, the presence of EphA2 facilitates the membrane insertion of TYPE7. Once TYPE7 binds to EphA2, there is a signaling cascade involving de-phosphorylation of Akt that inhibits cell migration as effectively as ephrinA1. However, the molecular mechanism used by TYPE7 to activate EphA2 involves a different pattern of phosphorylation of the intracellular domains, with large differences in juxtamembrane tyrosine phosphorylation. A second important difference is that while ephrinA1 induces EphA2 to assemble into large clusters, EphA2 bound to TYPE7 forms instead medium size oligomers. Taken together, our results show that TYPE7 inserts into membranes, where it interacts with the transmembrane and juxtamembrane region of EphA2. These interactions activate EphA2, and the activation loop of the kinase domain is phosphorylated, but not the juxtamembrane region. TYPE7 binding also causes oligomerization of EphA2. These results can have important implications for the design of new pH-triggered ligands that modulate protein-protein interactions in other membrane proteins.