Although it is a common species, almost nothing is known concerning the breeding biology of Yellow-margined Flatbill Tolmomyias assimilis. We present the first formal description of the species' nest. It was found in an open area near Manaus, Amazonas state, Brazil, and was, like the nests of other Tolmomyias species whose nests have been described, a closed structure, suspended from a branch, with an entrance tunnel.