© 2017 During tensile tests of carbon nanotube (CNT) macrostructures (ribbons, ropes and tows) under dynamic strain rates (1000 s –1 ), we recorded temporally sporadic, spatially localized visible light emissions (“flashes”) of less than 1.5 µs duration. The flashes occurred at the fracture sites and were, depending on the sample morphology, either distributed randomly over time (for tows) or occurred all at once over larger areas of several pixels (for ribbons). In situ thermal camera measurements, as well as ex situ analysis by electron microscopy reveal a hierarchical mechanism of overall heating over the whole sample length during straining, and localized heating around the fracture surfaces. Temperatures around the fracture tip were calculated to be of 1800 K in average. We propose that the flashes are caused by charge separation due to CNT bond fracture and gas discharge of the surrounding gases. Triboluminescence, known for larger sugar crystals, has not been observed for carbon nanotubes previously. It results from the yarn-like morphology, the ultra-high strength and thermal conductivity of our CNT fibers, which at high strain rates concentrate the strain at CNT level and lead to CNT fracture, rather than bundle sliding.