In this paper, a seven-step sequential extraction procedure (SEP) is proposed to evaluate As fractionation in environmental solid samples of concern. The protocol covers the most relevant As-bearing phases in soils and sediments, stressing the importance of the most mobilizable fractions, in accordance with their potential environmental hazards. The considered fractions are (1) readily soluble As, (2) strongly adsorbed As onto mineral surfaces, (3) As associated with Al oxyhydroxides, (4) As bound to organic matter, (5) As incorporated into amorphous Fe oxyhydroxides, (6) As associated with poorly crystalline Fe (hydr)oxides and (7) As coprecipitated with refractory minerals. Although this protocol is suitable for all kinds of environmental solid samples, it was especially designed for those that are highly polluted. Hence, optimization of some operational parameters in every extraction step such as the extraction time and solid-to-solution ratio was carried out by using highly polluted solids as control samples. The selectivity of the selected agents for every extraction step was assured through experiments with natural and synthetic matrices of As-bearing minerals and As-spiked materials. The validation of the proposed method was accomplished with five certified materials of soils and sediments covering a diverse range of As contents. The results of its application to As-contaminated samples from different geological origins and within a wide range of As contents revealed As fractionation to be largely in accordance with the pollution source of samples and with their geochemical composition, which confirmed this methodology to be suitable for fractionation assessment of As in environmental solid samples of different nature. In addition, one of the strengths of this method is that the simplicity of its steps would easily allow for its automation.