The electronic structures of [8]cycloparaphenylene dication ([8]CPP2+) and radical cation ([8]CPP center dot+) have been investigated by magnetic circular dichroism (MCD) spectroscopy, which enabled unambiguous discrimination between previously conflicting assignments of the UV-vis-NIR absorption spectral bands. Molecular orbital and nucleus-independent chemical shift (NICS) analysis revealed that [8]CPP2+ shows in-plane aromaticity with a (4n + 2) pi-electron system (n = 7). This aromaticity appears to be the origin of the unusual stability of the dication. Theoretical calculations further suggested that not only [8]CPP2+ but also all [n]CPP (n = 5-10) dications and dianions exhibit in-plane aromaticity.