Shigella sonnei is responsible for the majority of shigellosis infections in the US with over 500,000 cases reported annually. Here, we present the complete genome of the clinical multidrug resistant (MDR) strain 866, which is highly susceptible to bacteriophage infections. The strain has a circular chromosome of 4.85 Mb and carries a 113 kb MDR plasmid. This IncB/O/K/Z-type plasmid, termed p866, confers resistance to five different classes of antibiotics including ß-lactamase, sulfonamide, tetracycline, aminoglycoside, and trimethoprim. Comparative analysis of the plasmid architecture and gene inventory revealed that p866 shares its plasmid backbone with previously described IncB/O/K/Z-type Shigella spp. and Escherichia coli plasmids, but is differentiated by the insertion of antibiotic resistance cassettes, which we found associated with mobile genetic elements such as Tn3, Tn7, and Tn10. A whole genome-derived phylogenetic reconstruction showed the evolutionary relationships of S. sonnei strain 866 and the four established Shigella species, highlighting the clonal nature of S. sonnei.