Background: Standardized instruments are needed for rigorous research, quality clinical practice and informed policy-making. In Mongolia, a confluence of current social, demographic and geographic factors augment the need for reliable means to assess social isolation among older adults. This study aims to identify, translate and validate such a measure. Method: We conducted the study in two phases. We first selected the 18-item Lubben Social Network Scale and followed Brislin’s 6-step model to translate the instrument from English to Mongolian. The second phase involved psychometric testing of the Mongolian version (LSNS-18-M). We recruited 198 Mongolians aged ≥ 55 years from six hospitals in Ulaanbaatar. All participants also completed the SF-12; the 15-item Geriatric Depression Scale; and measures of objective and subjective Social Isolation’ of these, 27 completed a re-test of the LSNS-18-M.Results: The LSNS-18-M showed strong psychometric properties. Cronbach alpha coefficients were high for the overall scale (0.89) and for family (0.85), friend (0.91) and neighbor (0.89) subscales. Infraclass correlation coefficients for the total scale (0.99) and family (0.99), friend (0.98) and neighbor (0.99) subscales were also high in the test-retest subsample. With respect to convergent validity, LSNS-18-M scores were inversely related to Social Disconnectedness (r=-0.55, p<0.001)and Perceived Isolation (r=-0.65, p<0.001) and with the GDS-15 (r=-0.31, p<0.001). Although not related to either subscale of the SF-12, the LSNS-M-18 was inversely associated with self-rated health status (r=-0.27, p<0.01). Mean LSNS-18-M scores successfully differentiated three levels of Social Disconnectedness and three levels of Perceived Isolation, indicating good discriminate validity.Finally, content validity was good. Items on the LSNS-18-M loaded cleanly on the same three factors as the original LSNS-18. Inter-factor correlations ranged from 0.23 to 0.77, all three factors correlated with total LSNS-18-M scores at p<0.001, and together they accounted for the majority of variance in total LSNS-18 scores.Conclusion: The LSNS-18-M has excellent reliability and satisfactory validity and holds significant promise for assessing social isolation among older adults in Mongolia.