Resumo. This study reports the first occurrence of Spodoptera cosmioides (Walker, 1858) (Lepidoptera: Noctuidae) larvae causing defoliation in transplanted seedlings in a clonal plantation of Eucalyptus urophylla S.T. Blake (Myrtaceae), in Butiá municipality, Rio Grande do Sul State (30°9’45” S; 51°59’42” W). The S. cosmioides caterpillars are polyphagous, including eucalyptus among their host plants. It was observed that the caterpillar infestation damaged 80% of the seedlings, while the defoliation varied from 13% to 24% per plant. This is the first record of S. cosmioides caterpillars causing injuries in a new plantation of E. urophylla.