Four alkyl diamondoids, 1-pentyladamantane (PA), 2-butyladamantane (BA), 2-propyladamantane (2-PrA), and 2-ethyladamantane (EA), were synthesized on a preparative scale, and key fuel properties, including density, net heat of combustion, low-temperature viscosity, and derived cetane number (DCN), were measured. The fuel molecules had densities up to 17% higher than conventional jet fuel and volumetric net heats of combustion comparable to or exceeding that of the synthetic missile fuel JP-10. Remarkably, the alkyl diamondoid fuels had DCNs in the range of 42-49, which should allow for their efficient combustion in diesel engines. PA, BA, and 2-PrA all had essentially the same DCN (,49), while the short alkyl chain in EA lowered the value to 42.7. Bond dissociation energies for the formation of diradicals from 2-PrA and PA were calculated with density functional theory methods to gain a qualitative understanding of potential combustion mechanisms. Ring opening of the adamantane core during combustion is proposed to explain the high cetane number of these fuels. The alkyl diamondoids described in this report are the first examples of multicyclic hydrocarbons that combine extraordinary densities (>0.9 g/mL) with DCNs comparable to or exceeding that of conventional diesel fuel.