Quantitative description of the intrinsic steric and electronic properties of heteroaryl substituents is essential for establishing structure-activity relationships (SARs) and machine learning models for heteroaromatic compounds used in drug discovery and reaction design. DFT-computed descriptors are widely used in catalyst design , materials science , and reactivity and selectivity predictions . Descriptors for heteroaromatic compounds such as HOMO/LUMO orbital coefficients/energies and atomic charges have been applied to various reaction types, including electrophilic and nucleophilic aromatic substitution, and radical C-H functionalization. Despite these advances, a systematic and comprehensive database that integrates various physical-organic descriptors for heteroaryl substituents is still lacking, which has hindered the development of reactivity and selectivity prediction models. In contrast to the broadly used Hammett substituent constants (σp and σm) to describe electronic properties of aryl substituents, similar universal electronic descriptors for heteroaryl substituents have not been developed (Fig. ). This is in part due to the lack of experimental data (i.e., pKa values of corresponding heteroaryl carboxylic acids) as well as the inherent complexity of heteroaryl groups with different ring types, heteroatom substitutions, and regioisomers. We expect that a database of steric and electronic descriptors of heteroaryl substituents could serve as a foundation for developing robust predictive models that expand to the entire chemical space of heteroaromatic compounds. These could streamline reaction and catalyst developments by enabling the rational selection of heteroaryl substituents based on their electronic and steric features, rather than relying on trial-anderror approaches. In addition, this database for intrinsic chemical reactivity factors would complement existing cheminformatics databases for heteroarene synthetic feasibility and ADMET properties , which have been broadly used in drug discovery. Here, we present HArD, a HeteroAryl Descriptor database of >31,500 heteroaryl substituents based on 238 commercially available parent heteroarene cores (Fig. ). To capture the structural diversity of heteroaryl groups, we included both 5-and 6-membered heteroaromatic rings as well as 5,6-and 6,6-fused ring systems with carbon, nitrogen, oxygen, and sulphur as possible heavy atoms in the ring scaffold (Fig. ). Each parent heteroarene was functionalized with commonly used electron-withdrawing and electron-donating substituents to give monosubstituted heteroaryl groups (Fig. ). For each heteroaryl substituent, 49 DFT-computed electronic, steric, and geometrical descriptors and 16 fingerprint-type descriptors were included (Fig. and). This database includes computed Hammett-type substituent constants for heteroaryls (σHet), which would allow straightforward extensions of existing SAR and ML models of aryl compounds based on Hammett substituent constants (σp and σm) into previously unexplored space of heteroaryl-containing compounds. These newly developed σHet electronic parameters were computed based on pKa values of corresponding heteroaryl carboxylic acids (Fig. ), in analogy to the original definition of Hammett constants for aryl substituents to enable backward compatibility. In addition, other previously used descriptors, such as HOMO/LUMO coefficients, HOMO/LUMO energies, and partial atomic charges have also been computed for all heteroaryl groups in the database. Overall, HArD not only bridges a critical gap in the quantitative characterization of heteroaryl substituents but also provides a practical tool to design and predict the properties of these key building blocks in drug discovery, catalysis, and materials science.