La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) is considered a promising electrolyte material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) because of its high ionic conductivity. However, a main challenge in the application of LSGM is how to fabricate dense and thin LSGM membranes on electrode substrates at relatively low temperature (it is difficult to sinter LSGM to full density below 1500 °C). In this study, we report our findings on the preparation of thin LSGM electrolyte membranes using the low-cost atmospheric plasma spraying (APS) process. The phase composition, microstructure, and ionic conductivity of LSGM membranes deposited on an anode substrate depend sensitively on the particle size of LSGM powders because gallium (Ga) may evaporate during the APS process. When the particle size is <30 μm, Ga evaporation increases rapidly with the decrease in particle size, which may dramatically reduce the ionic conductivity of the LSGM deposits. For example, the ionic conductivity of an LSGM deposit was only ∼4.2% of the bulk conductivity, even though the deposits may be well bonded (with >80% inter-lamellar bonding ratio), as referred from the thermal conductivity measurement of the LSGM deposit (>80% of the bulk thermal conductivity). Using LSGM powders with particle sizes >30 μm, we have fabricated LSGM membranes having ionic conductivity of ∼0.075 S cm⁻¹ at 800 °C, ∼78% of the bulk value. Test cells based on plasma sprayed LSGM electrolyte membranes show excellent performance at 600–800 °C, suggesting that atmospheric plasma spraying is a promising approach for large-scale manufacturing of high-performance IT-SOFCs.