Amid global freshwater scarcity and soil salinization, brackish irrigation is a potential alternative, yet its effects under low-leaching soilless systems remain unclear. We tested brackish irrigation (30 mmol L⁻¹ NaCl; EC ≈ 4.8 dS m⁻¹, including fertilizer) on lettuce (Lactuca sativa L.) grown in peat–perlite substrates with non-saline (CK), mildly saline (M), and moderately–severely saline (S) initial salinity. Substrate moisture and bulk electrical conductivity (EC_b) were monitored at upper, middle, and deep layers with multi-depth sensors; lettuce physiological and growth traits were measured. Under negligible drainage, salt moved downward promptly after irrigation in CK, accumulated at the surface in M, and remained high with spatiotemporal variability in S. Brackish irrigation had minimal effects on biomass and water use efficiency in CK and M, but significantly reduced both in S. These findings support tailoring brackish irrigation to initial salinity severity and motivate future work to measure drainage and calibrate EC indices to establish operational thresholds.