In the present work, a procedure to determine three textile azo dyes, chromium-complexes [Nylosan Dark Brown (NDB), Lanasyn Dark Brown (LDB), and Lanasyn Red (LR)], from wastewater using solid-phase extraction (SPE) followed by LC-electrospray ionization negative mode tandem mass spectrometry (LC-ESI(-)-MS/MS) has been developed. The extraction/concentration and recovery degree of these dyes from liquid matrices were done on Strata WAX/NH2 cartridges. The chromatographic separation was performed using a Luna C18 (2) 100Å column by isocratic elution with a methanol-acetonitrile-water (0.2% formic acid and 2 mM ammonium formate) mixture. The linearity, the LOD, and the LOQ were determined for each textile dye. The accuracy, the precision (intra- and inter-day), and the matrix effect were also performed for the validation of the developed procedure. These chromium-complex azo dyes often used in the dyeing process by a textile factory in Romania were monitored in the influent and effluent wastewater from a treatment plant situated in its area. Applying the developed SPE-LC-ESI(-)-MS/MS procedure, the following textile dyes were detected (ng/L) in the collected wastewater samples during a 24-h period: NDB 150.1, LDB 200.6, and LR 89.0-244.0 in influents and NDB 22.8, LDB 78.6, and LR 74.0 in effluents.