Diabetes mellitus is a complex set of conditions that impacts 34 million Americans. While type 1 diabetes, type 2 diabetes, and gestational diabetes are most frequently encountered, there are many other types of diabetes with which healthcare providers are less familiar. These atypical forms of diabetes make up nearly 10% of diabetes cases and can masquerade as type 1 or 2 diabetes mellitus (T1DM or T2DM), and the treatment may not be optimized if the diagnosis is not accurate. Atypical forms include monogenic diabetes (formally known as maturity-onset diabetes of the young [MODY]), latent autoimmune diabetes of the adult (LADA), ketosis-prone diabetes, and secondary diabetes. This paper will detail the defining characteristics of each atypical form and demonstrate how they can masquerade as type 1 or 2 diabetes mellitus. Gestational diabetes mellitus will not be discussed in this article.