BACKGROUND Group G streptococcus (GGS) infection is reported to have invasive pathogenicity similar to that of group A streptococcus (GAS) infection, causing a strong systemic inflammatory response with bacteremia and various complications. Herein, we report a case of posterior reversible encephalopathy syndrome (PRES) as a rare complication of a GGS infection. CASE REPORT An 89-year-old Japanese man presented to our hospital with gastrointestinal bleeding and shoulder pain. Close examination revealed a refractory duodenal ulcer (DU) with disseminated intravascular coagulation and soft tissue infection of the right arm, which was found to be caused by GGS. A hemorrhagic tendency due to disseminated intravascular coagulation made it difficult to achieve hemostasis, leading to repeated blood transfusions. Although remission of both the DU and infection was achieved with treatment, impairment of swallowing function and vision subsequently appeared. Magnetic resonance imaging revealed hyperintense lesions with elevated apparent diffusion coefficient (ADC) values on T2-weighted imaging (T2WI), fluid-attenuated inversion recovery (FLAIR), and diffusion-weighted imaging (DWI). The patient was diagnosed with PRES, which did not improve even after discharge on day 118. CONCLUSIONS GGS infection developed with refractory duodenal ulcer bleeding, resulting in PRES with irreversible sequelae. The occurrence of PRES, which may be a rare complication of GGS infection, should be considered when central nervous system manifestations are observed in case of invasive streptococcal infection with a systemic inflammatory response.