A new azo-dye reagent was prepared by the reaction between sulfacetamide and rutin compounds. The new synthesized reagent of (E)-N-((4-((5-(5,7-dihydroxy-3-3-[3-α-l-rhamnopyranosyl-(1 → 6)-β-d-glucopyrano-syloxy]-4-oxo-4H-chromen-2-yl)-2,3-dihydroxyphenyl)diazenyl)phenyl)sulfonyl)acetamide [NDRGA] was characterized by FT-IR, 1 HNMR, mass spectra and elemental analysis measurements and then used for the spectrophotometric determination of Sm(III). The proposed method was based on the formation of a lemon-colored complex between Sm(III) and NDGRA reagent in an alkaline medium using borate bufer at pH = 8 with absorption maximum at 475 nm. The method was enhanced by the use of cationic surfactant of cetylpyridinium bromide (CPB). Diferent factors afecting the formation and stability of the complex such as reagent concentration, time, temperature, solvents and order of addition were also studied. The composition of the com-plex was found to be 1:1 (metal: ligand) by using Job’s and molar ratio methods. The stability constant of the complex was calculated to be 1.1805 × 10 6 . The method showed a good linearity in the concentration range of 2.0–90 µg ml −1 of Sm(III) with molar absorptivity and Sandell’s sensitivity 1.3014 × 10 4  L mol −1  cm −1 and 1.155 × 10 −2  µg cm −2 , respectively. The limit of quantifcation (LOQ) and detection (LOD) were calculated. The interference efect of some foreign ions was also studied. The validity of the calibration curve was found useful for the determination of micro-amounts of Sm(III) in some industrial and blood samples.