The Doppler effect is a fundamental frequency shift phenomenon that occurs whenever a wave source and an observer are moving with respect to one another. It has well-established applications in astrophotonics, biological diagnostics, weather and aircraft radar systems, velocimetry and vibrometry. The counterintuitive inverse Doppler effect was theoretically predicted in 1968 by Veselago1 in negative-index materials2. However, because of the tremendous challenges of frequency shift measurements inside such materials, most investigations of the inverse Doppler effect have been limited to theoretical predictions and numerical simulations3, 4, 5, 6, 7. Indirect experimental measurements have been conducted only in nonlinear transmission lines at ~1–2 GHz (ref. 8) and in acoustic media at 1–3 kHz (ref. 9). Here, we report the first experimental observation of the inverse Doppler shift at an optical frequency (λ = 10.6 µm) by refracting a laser beam in a photonic-crystal prism that has the properties of a negative-index material.