Silica nanoparticles (SNPs) were prepared via a sol-gel method and were functionalized using cyanex921 (a Tri-octyl phosphine oxide phosphorous containing ligand) to yield the modified CY-SNPs particles. The synthesized SNPs and CY-SNPs were characterized by using SEM, FT-IR, EDS, XRD, Zeta Potential and thermogravimetric (TGA/DTA) analyses. The formed CY-SNPs were successfully used to selectively adsorb the methylene blue basic dye (MB) from real water samples in batch mode. The effect of pH, temperature, amount of sorbent, contact time, interfering ions and initial amount of MB on the adsorption capacity of CY-SNPs were examined. The chemical adsorption was shown to be the rate-determining step according to the second-order kinetic model. Additionally, the adsorption procedure showed the best fit with Langmuir model with maximum adsorption capacity of MB of 161.9 mg g⁻¹. Desorption of more than 95 % of the adsorbed dye was obtain using 5 mL of dehydrated ethanol. Also, the sorbent can be re-used for 3 times with undiminished adsorption capacity. The application, reusability indicated that CY-SNPs sorbent have a great potentiality for the removal of basic dyes from various water samples. The mechanism of adsorption of the basic dye onto the phosphorus modified Silica Nanoparticles was discussed.