Helical structures are ubiquitous in natural and synthetic materials across multiple length scales. Excellent and sometimes unusual chiral optical, mechanical, and sensing properties have been previously demonstrated in such symmetry-breaking shape, yet a general principle to realize helical structures at the sub-100 nm scale via colloidal synthesis remains underexplored. In this work, we describe the wet-chemical synthesis of monodisperse nanohelices based on gadolinium oxide (Gd2O3). Aberration-corrected electron microscopy revealed that individual nanohelices consist of a bilayer structure with the outer and inner layers derived from the {111} and {100} planes of bulk Gd2O3, respectively. Distinct from existing inorganic nanocoils with flexible bending geometries, the built-in lattice misfit between two adjacent crystal planes induces continuous helical growth yielding three-dimensional rigid nanohelices. Furthermore, the presence of water in the reaction was found to suppress the formation of nanohelices, producing nanoplates expressing predominantly {111} planes. Our study not only provides a bottom-up synthetic route and mechanistic understanding of nanohelices formation but may also open up new possibilities for creating chiral plasmonic nanostructures, luminescent biological labels, and nanoscale transducers.