Here we describe Iranichnus farsensis igen. et isp. nov., a bioerosion trace in fossil wood characterized by a system of sinuous channels in the wood under the bark, also bearing small borings radially oriented within the channels. We attributed this to insects, most probably a beetle from one of the groups known to feed on wood just under the bark: Curculionidae (Scolytinae), Cerambycidae, Buprestidae, Bostrichidae, or Ptinidae. The fossil log is from the Siliciclastic Unit of the Upper Cretaceous Tarbur Formation in the Zagros Basin, southwestern Iran, being the first reported occurrence of fossil logs in this unit. The Tarbur Formation represents an essentially marine depositional system, but the fossil logs indicate a continental palaeoenvironment from which the log was brought. Since the most probable trace-makers are terrestrial insects, the trace fossil was produced when the log was still subaerially exposed. Insect body fossils are scarce, and studying their trace fossils, mainly those from the Late Cretaceous, is fundamental to understanding the aftermath of the insect Cretaceous diversification, and its consequences in the Cenozoic.