Steelmaking slag, a by-product of the steel-refining process, could be used for removing boron excess from irrigation natural and waste waters, due to its strongly alkaline reaction. The objectives of this study were to: (a) establish the optimum conditions (external solution/adsorbent ratio, equilibration time) of boron adsorption by the slag, (b) assess the slag’s capacity to adsorb boron, and (c) study boron desorption from the slag with time. Boron adsorption increased with the increase of the external solution/adsorbent ratio up to the ratio of 200:1. Although, almost 40% of boron was adsorbed within the first hour of equilibration period, the adsorption gradually increased until the 72 h. The Langmuir adsorption maximum for boron was 145 mg g−1, considerably higher than other adsorbents, like fly ash, calcite, and magnesia. At boron initial concentrations lower than 6 mg L−1, slag removed 55% of boron and reduced it below the permissible levels for irrigation waters (< 4 mg L−1) for most crops. The pH of the equilibrium solution was 10.3 ± 0.8 and dropped to acceptable levels for irrigation waters (< 8.5), after contact with atmosphere for 1 week. Almost 25% of boron was released from samples of boron-laden slag during the first hour of desorption. Consequently, steelmaking slag can be used effectively for removing boron excess from irrigation waters. However, attention should be given to the pH of the slag-treated waters. Furthermore, the disposal of boron-laden slag to soils should be practiced with caution to avoid possible boron phytotoxicity risk.