Despite over a hundred years of intense paleontological exploration, the terrestrial rocks of the Upper Cretaceous (Maastrichtian) Hell Creek Formation of eastern Montana are remarkable for the absence of fossil eggs. Here, we describe the first fossil egg and additional eggshell fragments from the formation. The two-layered structure of the smooth 1600-µm-thick eggshell of Belonoolithus garbani, oogen. et oosp. nov., permits assignment of the 6 cm × 8 cm egg to Theropoda. Additional theropod eggshells, Dimorphoolithus bennetti, oogen. et oosp. nov., closely resemble Tubercuoolithus choteauensis (incertae sedis) from the Campanian Two Medicine Formation of Montana, differing only in ornamentation. Therefore, we establish Tubercuoolithidae, oofam. nov., to accommodate both eggshell types. Other eggshell types include two varieties of Spheroolithus, as well as Testudoolithus and Krokolithes. Whereas B. garbani and D. bennetti are unique to the Hell Creek Formation, the other three oogenera also occur in the stratigraphically older Two Medicine and Judith River formations of Montana, and the Oldman Formation of southern Alberta, Canada.