Among amniotes, turtles are the only clade that lay aragonitic eggs. Because aragonite is a metastable mineral, unequivocal preservation of aragonite in fossil turtle eggs has only been reported from Pliocene deposits. Here, we report in situ preservation of aragonite in a turtle egg from the Judith River Formation (Campanian) of Montana, USA. We utilized electron backscatter diffraction (EBSD) and Raman spectroscopy to explore the carbonate mineral polymorphs in the eggshell. The EBSD maps show the presence of both aragonite and calcite with the former preserved as needle-like crystal, a feature of all turtle eggshells. The presence of aragonite is independently validated by Raman maps. This dual approach provides direct, unambiguous evidence that the record of aragonitic turtle eggshell dates back at least to the Campanian (ca. 76 Ma) and supports the hypothesis that aragonitic eggshell is a synapomorphy of all turtles. The presence of pristine aragonite also indicates minimal taphonomic alteration at the fossil locality. Consequently, diverse invertebrate or vertebrate fossils from localities with aragonitic turtle eggs may provide high quality, unaltered palaeoenvironmental information.