S-1 plus cisplatin is the standard chemotherapy for recurrent gastric cancer. While depression and delirium are frequent in cancer patients, hypomania during chemotherapy is rare. We describe a rare case of hypomania during S-1 plus cisplatin treatment for recurrent gastric cancer. A 66-year-old woman, with no previous psychiatric disorder, received S-1 plus cisplatin for recurrent gastric cancer. She showed peculiar behavior. Physical examination, urine, blood and imaging findings were normal. There was no gastric cancer progression. During psychiatric consultation, she behaved inappropriately. However, she behaved normally while performing daily activities. She manifested a persistently elevated, expansive or irritable mood, clearly different from her usual non-depressed state, meeting hypomania diagnostic criteria. Her condition did not require chemotherapy discontinuation or additional medication. During the second and subsequent S-1 plus cisplatin cycles, symptoms were stable. Cancer patients often have adjustment disorders, depression and delirium, but rarely hypomania. Our patient showed no significant changes in blood biochemistry and brain and whole body imaging. While S-1 plus cisplatin-induced hypomania cannot be excluded, hypomanic symptoms did not improve during the chemotherapy rest period, nor was there deterioration during subsequent cycles, suggesting drug-induced mania to be unlikely. Possible onset mechanisms include manic defense phenomena, common with stressful life events. There are no reports of recurrent gastric cancer patients experiencing hypomania during S-1 or S-1 plus cisplatin therapy, i.e. our patient represents a rare course. Clinicians should recognize psychosis or mood disorders during gastric cancer treatment. Further accumulation of such rare cases might elucidate pathological mechanisms underlying hypomania in cancer patients.