The synthesis, structure, and reactivity of an actinide metallacyclopropene were comprehensively studied. The reduction of [eta(5)-1,2,4-(Me3C)(3)C5H2](2)ThCl2 (1) with potassium graphite (KC8) in the presence of diphenylacetylene (PhC=CPh) yields the first stable actinide metallacyclopropene [eta(5)-1,2,4-(Me3C)(3)C5H2](2)Th(eta(2)-C2Ph2) (2). The magnetic susceptibility data show that 2 is indeed a diamagnetic Th(IV) complex, and density functional theory (DFT) studies suggest that the 5f orbitals contribute to the bonding of the metallacyclopropene Th-(eta(2)-C-C) moiety. Complex 2 shows no reactivity toward alkynes, but it reacts with a variety of heterounsaturated molecules such as aldehyde, ketone, carbodiimide, nitrile, organic azide, and diazoalkane derivatives. DFT studies complement the experimental observations and provide additional insights. Furthermore, a comparison between Th and group 4 metals reveals that Th4+ shows unique reactivity patterns.