Under proper conditions, the response of two coupled nonlinear oscillators, driven by a sinusoidal input, contains a pair of fractional‐harmonic frequency components. The frequenciesf1andf2of the fractional‐harmonic pair correspond approximately to the resonance frequencies of the system, and the sumf1+f2equals the driving frequency. Conditions for the occurrence of fractional harmonics are examined by an analysis that relates them to a pair of coupled Mathieu equations. Fractional‐harmonic components were observed in several nonlinear electrical, mechanical, and acoustical systems. The observations are consistent with qualitative predictions of the analysis.