The ammonites in spherical carbonate concretions often preserve their original three-dimensional (3-D) shell shapes and detailed fragile structures. However, the formation process of spherical ammonite concretions is not fully understood. Herein, the ammonite concretions identified in the Cretaceous (Campanian) Osoushinai Formation, Yezo Group, Japan, are examined to understand their formation process during soft-tissue decomposition after burial in marine sediments. In the Osoushinai Formation, almost all observed ammonites in concretions preserve their 3-D form without phragmocone deformation. The calcite filling in the remaining body chamber of ammonites (BC1) shows that shells were buried with soft tissues. These occurrences, negative δ13C values, and the near-zero δ18O values of BC1 as well as the concretions indicate that both BC1 and concretions rapidly formed from dissolved inorganic carbon derived from organic matter, including the soft tissue of dead organisms, in the shallow part of the sediments. The increasing Fe concentration in BC1 shows that BC1 formed in the iron-reduction (FeR) zone, where organic matter was decomposed owing to the activity of iron-reducing microorganisms. The similarity of the elemental and isotopic compositions of BC1 and concretions show that they concurrently formed in the FeR zone. In the Osoushinai Formation, an abundant influx of Fe(III) and intense bioturbation during the deposition of the formation promoted organic decomposition in the FeR zone, causing rapid formation of BC1 and concretions. Such rapidly formed calcite fillings and concretions protected fossils from deformation and dissolution during diagenesis to preserve their 3-D form. Overall, the findings of this study provide new insight into the relation between sedimentary environments and the fossil preservation process via rapid concretion formation.