Lithium has proven its efficacy in treating bipolar disorder. Severe side effects caused by lithium, including renal and endocrine outcomes, have already been amply documented. The impact of lithium on sexual function, however, is less well known. A 33-year-old man, with no past medical history, diagnosed with bipolar disorder, developed premature ejaculation after short-term use of lithium. The dose of lithium was reduced, leading to a rapid clinical resolution. Retrospectively, lithium-induced premature ejaculation was deemed the most likely diagnosis. Premature ejaculation is a rare side effect of lithium. Changing the time of medication administration and lowering dose could be considered as alternatives. Given lithium's pharmacological profile, it is likely that the pathophysiologic mechanism behind premature ejaculation is altered levels or altered serotonin receptor sensitivity in the ejaculatory modulating centers of the central nervous system. Given the reluctance to spontaneously report sexual adverse effects, clinicians should be aware of this possible side effect.