This case report describes methemoglobinemia in a dog suspected to be the result of consumption of a large volume of fermented bok choy. The patient presented with clinical signs and co-oximetry consistent with methemoglobinemia without ingestion of a known toxin. A large volume of fermented bok choy had been ingested earlier that day and decontamination procedures were performed as a result. Supportive care led to resolution of clinical signs and appropriate clearance of methemoglobin. While erythrocyte oxidant damage is a consequence of ingestion of plants in the genus Brassica (such as bok choy) in ruminant species due to rumen microbiota producing sulfur-containing compounds, specifically dimethyl disulfide, there are potential pathways that can lead to similar effects in monogastric animals. The methemoglobin formation in this patient may have resulted from the large volume consumed with the natural fermentation releasing dimethyl disulfide and leading to oxidant damage analogous with that in ruminants. This case report provides additional mechanisms for methemoglobin formation in dogs and to direct the clinician toward methemoglobinemia in patients with compatible clinical signs with ingestion of specific plant species.