The present work focused on the potential use of brown algae Cystoseira barbata and Cystoseira crinita from the Black Sea coast for removal and speciation analyses of Cr(III,VI) ions from aqueous and wastewater solutions. The biosorption process of Cr(III) and Cr(VI) was designed as a function of pH and contact time. Potentiometric titration and Fourier transform infrared spectroscopy (FT-IR) analysis techniques revealed the potential binding sites present at the surface of the algae for both oxidation states of Cr. Various chemical treatments have been used to indicate the mechanisms of binding Cr(III,VI) and bioreduction of Cr(VI) by the biosorbents. Acidic treatment was the most successful in removing and reducing total Cr(VI). Algae samples were subjected to methylation and esterification processes for modification of amino and carboxyl groups, respectively. The Langmuir model was applied to describe the biosorption of Cr(III,VI) by algae. Total Cr and Cr(VI) determinations were simultaneously made using the diphenyl carbazide spectrophotometric method and flame atomic absorption spectroscopy (FAAS). In conclusion, these algae can be used as a potentially cost-effective biosorbent for the uptake of two different oxidation states of Cr and subsequently for Cr speciation analysis.