There are many reasons why there is a sudden mole-like black spot on one side of the tongue, such as bruises, oral pigmented moles, hairy tongue and so on. 1. Bruise point: local friction, trauma and other factors can lead to bruise points on the tongue, which can cause the phenomenon of a sudden mole-like black spot on one side of the tongue. 2. Oral pigmented nevus: a sudden nevus-like black spot on one side of the tongue may belong to pigmented nevus, which is formed by pigment cells in the oral cavity and has a high probability of malignant change compared with pigmented nevus in other parts of the body, so it needs to be dealt with in time. 3. Hairy tongue: a sudden mole-like black spot on one side of the tongue, if the area is large, it may belong to the hairy tongue, especially after eating food containing pigment, long-term smoking symptoms will be more serious. There are many reasons why there is a mole-like black spot on one side of the tongue, it is recommended to go to the hospital in time to improve the examination, after a clear diagnosis and take timely treatment, so as not to delay the condition.