Abstract Methods of designing steel monosymmetric I-beams against lateral buckling are not well supported by research. For this paper, the inelastic buckling of monosymmetric steel I-beams under moment gradient is studied and compared with design recommendations. For welded beams in uniform bending, inelastic buckling is initiated at moments which are often close to those which cause first yield in the compression flange. Once initiated, the inelastic buckling resistance remains constant as the slenderness decreases until strain-hardening occurs. For hot-rolled beams in uniform bending, the inelastic buckling resistance increases almost linearly as the slenderness decreases. Three regimes are significant in the inelastic buckling resistances of hot-rolled monosymmetric beams under moment gradient, depending on which flange yields first and the end moment ratio. Simple linear approximations of good accuracy are developed for designing hot-rolled monosymmetric beams in uniform bending, while less accurate but conservative approximations are developed for moment gradient. The use of these approximations is illustrated by a worked example.