The study investigates the ultrafast structural dynamics of cyclobutanone after photoexcitation at a wavelength of 200 nm, utilizing gas-phase megaelectronvolt ultrafast electron diffraction. This research complements simulation studies on the same topic. The experimental approach provides data on both electronic state population and structural dynamics through distinct inelastic and elastic electron scattering signatures. Key findings from the experiment include: The photoexcited S2 state of cyclobutanone is depopulated with a time constant of (0.29 ± 0.2) ps, transitioning towards the S1 state. The S1 state population undergoes a Norrish Type-I ring-opening reaction. The structural changes associated with this appear with a delay of (0.14 ± 0.05) ps after the initial photoexcitation. The resulting biradical species further reacts within (1.2 ± 0.2) ps through two competing fragmentation channels, producing either ketene and ethylene, or propene and carbon monoxide. This research highlights the utility of gas-phase ultrafast diffraction as a benchmark for nonadiabatic dynamics simulation methods.