The electronic structure, stability, chemical bonding and mechanical properties of 3d, 4d and 5d transition metal diboride TMB2 were investigated using first-principles calculations based on density functional theory. All the primary chemical bonds, i.e., metallic, ionic and covalent have contributions to the bonding of TMB2. The number of valence electrons of transition metals or the valence electron concentration (VEC) of TMB2 has strong effects on the lattice parameters, stability and mechanical properties of TMB2. Both lattice constants a and c decrease with VEC, but c decreases faster than a, which is attributed to the enhanced TM d–B p (sp2) bonding. Bulk modulus B of TMB2 increases continuously with VEC due to the enhanced TM d–B p (sp2) and TM dd bonding. Shear modulus G increases with VEC, reaching a maximum at VEC = 3.33, and then decreases with further increase of VEC. YB2 and MnB2 have low Young's modulus and are predicted to have good thermal shock resistance. According to Pugh's criterion (G/B < 0.571), MnB2, MoB2 and WB2 are predicted as ductile or damage tolerant ultrahigh temperature ceramics (UHTCs).