Biosorption offers cost advantages over conventional treatment methods due to technical and financial constraints. In present paper, calcium alginate (CaAlg) monolithic pellets are investigated as biosorbents for the removal of methylene blue (MB) from aqueous solution in a batch mode. Direct and indirect ultrasonication was employed for the preparation of porous CaAlg pellets to enhance their removal efficiency (R%) and adsorption capacity (qe). In preliminary stage, the effectiveness of pure CaAlg pellets were evaluated based on different concentrations of sodium alginate (SA) (1wt%, 2wt%, and 3wt%). The synthesis of CaAlg pellets from 3wt% sodium alginate exhibited the highest adsorption capacity of 1.302 mg/g. Surface characterizations were studied to examine the morphology of the pure and ultrasonicated 3wt% CaAlg monolithic pellets using scanning electron microscopy (SEM) before and after adsorption. SEM images revealed that the surface of ultrasonic-assisted CaAlg monolithic pellets exhibits pores and fissures. Meanwhile, the experimental results indicated that the R% and qe of CaAlg pellets arranged in a descending order of direct ultrasonicated CaAlg (88.13%, 2.058 mg/g) > indirect ultrasonicated CaAlg (78.61%, 1.657 mg/g) > pure CaAlg (57.06%, 1.302 mg/g) at a batch adsorption condition of 250 rpm at 30 °C for 6 h. These pellets performed at least 21% better for MB removal than those synthesized in the absence of ultrasonic waves. Thus, the adoption of ultrasonication showed a promising potential for the synthesis of CaAlg monolithic pellets with improved adsorption properties to remove organic pollutants from dye wastewaters.