We observed Trumpeter Swans (Cygnus buccinator) nesting on top of an active muskrat (Ondatra zibethicus) lodge, indicating simultaneous breeding uses of a constructed vegetation mound not previously recorded in the literature. Specifically, within the Red Rock Lakes National Wildlife Refuge in southwestern Montana, we observed 4 eggs in an actively tended Trumpeter Swan nest, while underneath the swan nest bowl was a muskrat nursery occupied by 3 neonate muskrats. Swans are known to use abandoned muskrat lodges as nest foundations, but not active lodges. Our observations point to a close association between breeding muskrats and Trumpeter Swans in the shallow wetlands of North America and emphasize the benefit of robust muskrat populations for Trumpeter Swan restoration.