The brown planthopper, Nilaparvata lugens (Stål), is a major rice pest in Asia. Acetamiprid is a widely used neonicotinoid insecticide for N. lugens control. Because of the frequent and excessive applications of chemical insecticides, moderate levels of resistance against acetamiprid have been found in field populations of N. lugens in China. To study the inheritance mode and fitness costs of acetamiprid resistance in N. lugens, an acetamiprid-resistant strain (AC-R) was selected from a susceptible strain (AC-S) in the laboratory. After 28 generations of selection, AC-R showed 26.15-fold resistance compared to AC-S. The LC₅₀ and degree of dominance (D) values of the F₁RS and F₁SR (progenies of reciprocal crosses) indicated that the acetamiprid resistance in N. lugens was inherited as autosomal and incompletely dominant. Significant differences between the expected and observed mortalities of the F₂ individuals suggested that acetamiprid resistance is controlled by multiple genes. The AC-R strain showed a relative fitness of 0.71 with a significantly decreased male adult longevity, fecundity, net reproductive rate (R₀), and finite rate of increase (λ), and a prolonged adult preoviposition period (APOP). However, the durations of the 1st, 2nd, and 4th instar nymphs of AC-R were significantly shorter than those of AC-S. The current study provides valuable information for understanding acetamiprid resistance and optimizing insecticide resistance management strategies of N. lugens in the field.