A 7-year-old Korean shorthair cat was admitted to our hospital with chronic constipation. Abdominal ultrasonography incidentally revealed a focal asymmetric gastric mass. The mass was submucosal and hypoechoic without loss of wall layering. Histopathological examination revealed a gastric submucosal lipoma (GSL). Although there have been reports of gastric submucosal fat infiltration in cats, there have been no reports regarding GSL. To our knowledge, this is the first report describing the ultrasonographic characteristics of GSL in a cat. Gastric submucosal lipoma should be considered as a differential diagnosis when a focal hypoechoic submucosal mass without loss of wall layering in the stomach is observed on ultrasound images. Key clinical message: This case report describes the ultrasonographic characteristics of GSL in a cat and aims to provide useful information for the diagnosis of lipoma occurring in the feline gastrointestinal tract. The ultrasonographic features and histological results we describe should be helpful in diagnosing submucosal lipoma in cats with similar conditions.