Lean Six Sigma (LSS) combines waste reduction with process variation control, but adoption by manufacturing small and medium-sized enterprises (SMEs) may be constrained by limited resources, weak management support, skills shortages, and low familiarity with structured improvement methods. This study examines the relationships among barriers to LSS implementation, critical success factors (CSFs), LSS implementation practices, and operational performance among manufacturing SMEs in South-Eastern Nigeria. A quantitative cross-sectional questionnaire survey was conducted across Owerri, Aba, Onitsha, and Nnewi. Of 242 questionnaires distributed, 184 valid responses were retained for analysis. The instrument contained 24 construct items covering barriers, CSFs, LSS implementation practices, and operational performance. SmartPLS 4.0 was used to assess the measurement and structural models, with significance evaluated using 5,000 bootstrap resamples. The reported results indicate a negative relationship between barriers and LSS implementation (β = -0.412, t = 6.124, p < .001), a positive relationship between CSFs and LSS implementation (β = 0.587, t = 9.452, p < .001), and a positive relationship between LSS implementation and operational performance (β = 0.680, t = 11.210, p < .001). The model explains 51.2% of the variance in LSS implementation (R² = 0.512) and 46.2% of the variance in operational performance (R² = 0.462). The findings support a practical implementation pathway for resource-constrained SMEs that combines leadership awareness, low-cost Lean tools, simple data collection, a focused pilot project, and routine improvement reviews. Given the cross-sectional design, the estimated relationships are interpreted as associations rather than proof of causation. The measurement statistics and original SmartPLS outputs should be retained for verification and auditability.