Interactions between the sigma1 receptor agonist PRE-084 and various lipid monolayers, including dipalmitoylphosphatidylcholine (DPPC), DPP-ethanolamine (DPPE), DPP-glycerol (DPPG), DPP-serine (DPPS), palmitoylsphingomyelin (PSM), and cholesterol (Ch), were investigated to elucidate the effects of PRE-084 on membrane fluidity and stability. Their interactions with sigma1 receptor agonists have potential implications for neuroprotection, antidepressant, analgesic, and cognitive enhancement effects. In this study, we observed that the presence of PRE-084 in the subphase led to increased fluidity in DPPC and DPPE monolayers, whereas decreasing fluidity was observed in DPPG, DPPS, and PSM monolayers. The interaction of PRE-084 with Ch monolayers was found to be distinct from its interaction with other lipids. Fluorescence microscopy images revealed changes in the size and shape of liquid-condensed domains in the presence of PRE-084, supporting the notion of altered membrane fluidity. Our findings provide new insights into the interaction of PRE-084 with lipid monolayers and its potential implications for biological and membrane science.