A simple, rapid, sensitive, and economical spectrophotometric method for the determination of erbium Er(III) ions using 3-[2-hydroxy-3-sulfo-5-nitrophenylazo]-pentadione-2,4 (R) in the presence of cetyltrimethylammonium bromide (CTMABr) as a cationic surfactant is described. Optimal complexation conditions (λopt, pHopt) for the homogeneous (Er(III)-R) and mixed-ligand (Er(III)-R-CTMABr) complexes are determined. Er(III)-R complex exhibits maximum absorption capacity at 417 nm, while Er(III)-R-CTMABr complex exhibits maximum absorption capacity at 426 nm. The molar absorption coefficient for the Er(III)-R complex is 0.92∙104, and that for Er(III)-R-CTMABr complex is 1.78 ∙ 104. The optimal pH value for Er(III)-R is 8, and for Er(III)-R-CTMABr complex it is 6. In the presence of CTMABr, the optical density of the complex solutions increases significantly, and the optimal pH value for complex formation shifts to the acidic region. The region of conformity to Bouguer–Lambert–Beer’s law is linear in the range of erbium concentrations from 1.34 to 5.30 μg/mL for Er(III)-R and from 0.62 to 6.65 μg/mL for Er(III)-R-CTMABr. The stoichiometric ratio of the components in the complexes was confirmed using isomolar series, Starik–Barbanel relative yield, and shift method. These methods confirmed the formation of complexes with component ratios of Er(III):R = 1:2 and Er(III):R:CTMABr = 1:1:1. The influence of certain foreign ions on the formation of Er(III) complexes was studied. The proposed method was used to determine trace amounts of Er(III) in volcanic rocks.