During Dictyostelium development, prespore cells secrete acyl-CoA binding protein (AcbA). Upon release, AcbA is processed to generate a peptide called spore differentiation factor-2 (SDF-2), which triggers terminal differentiation of spore cells. We have found that cells lacking Golgi reassembly stacking protein (GRASP), a protein attached peripherally to the cytoplasmic surface of Golgi membranes, fail to secrete AcbA and, thus, produce inviable spores. Surprisingly, AcbA lacks a signal sequence and is not secreted via the conventional secretory pathway (endoplasmic reticulum-Golgi-cell surface). GRASP is not required for conventional protein secretion, growth, and the viability of vegetative cells. Our findings reveal a physiological role of GRASP and provide a means to understand unconventional secretion and its role in development.