A 35-year-old immunocompetent woman from southern China went to the hand surgery clinic with a six-month history of progressive swelling in her right index finger. She had been pinched by a lobster and had received several treatments without any improvement. Pus specimens were taken from the swollen parts of her finger, and the pathology showed granulomatous inflammation. Ziehl-Neelsen staining revealed positive bacillus in the pus specimens. The bacteria grew well on Columbia blood agar. However, the MALDI-TOF MS and 16S rRNA gene sequencing were not able to distinguish between Mycobacterium marinum and Mycobacterium ulcerans because of their close genetic relationship. Photochromogenicity testing can help differentiate between these species based on the alteration in colony color after light exposure. For our patient, the colonies turned yellow after 18h of incubation in the sun, identifying the species as M. marinum. Besides surgical drainage, the patient received rifampicin and clarithromycin for three months, and her symptoms resolved without relapse after six months of follow-up.