[Objectives]  To investigate the genetic diversity and population genetic structure of the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), an important polyphagous pest in China, that causes serious economic losses in Liaoning province, Northeast China and other crop-producing areas. [Methods]  The genetic variation, genetic differentiation, gene flow and population genetic structure of 160 S. exigua specimens from 6 geographic populations in Liaoning province were investigated by analyzing the genetic variation among nine microsatellite (SSR) loci with GeneAlex 6.51, GENEPOP 4.0.1 and STRUCTURE 2.3.4. [Results]  Populations had a high overall level of genetic diversity (Na=6.104, Ho=0.549, Ne=3.431, He=0.608); the population with the most diversity was the Shenyang (SY) population (Na=8.125, Ho=0.563, Ne=4.562, He=0.680). There was a low level of genetic differentiation among populations; the greatest genetic differentiation (F_ST=0.210) was between the Lingyuan (LY) and Dalian (DAL) populations. UPGMA, PCoA and STRCTURE analysis indicate that S. exigua populations in Liaoning can be divided into two groups; the first comprised of the Huludao (HLD) and Dalian (DL) populations and the second of the Tai'an (TIA), Fuxin (FX), Lingyuan (LY) and Shenyang (SY) populations. AMOVA analysis indicates that most (78.0 %) genetic variation in S. exigua in Liaoning was within populations and that inter-population variation was low (22.0 %). [Conclusion]  There is relatively high genetic diversity and low level of genetic differentiation in geographical populations of S. exigua in Liaoning province. These findings help resolve the genetic structure of S. exigua in Liaoning and thereby contribute to the development of better management strategies for this pest.