Herein, we report for the first time the use of ternary oxide nanoheterostructure photocatalysts derived from (Nby, Ti1–y)2CTx MXene in the treatment of water. Three different compositions of binary MXenes, viz., (Ti0.75Nb0.25)2CTx, (Ti0.5Nb0.5)2CTx, and (Ti0.25Nb0.75)2CTx (with Tx = OH, F, and Cl), were used as single-source precursor to produce TiNbOx-3:1, TiNbOx-1:1, and TiNbOx-1:3 by controlled-atmosphere thermal oxidation. Phase identification and Le Bail refinements confirmed the presence of a mixture of rutile TiO2 and monoclinic Ti2Nb10O29. Morphological investigations through scanning and transmission electron microscopies revealed the retention of layered nanostructures from the MXene precursors and the fusion of TiO2 and Ti2Nb10O29 nanoparticles in forming nanosheets. Among the three oxide nanoheterostructures, TiNbOx-3:1 exhibited the best photocatalytic performance by the removal of 83% of sulfamethoxazole (SMX) after 2 h of reaction. Such a result is explained by a complex influence of structural, morphological, and electronic properties since TiNbOx-3:1 consisted of small-sized crystallites (40–70 nm) and possessed a higher surface area. The suggested electronic band structure is a type-II heterojunction, where the recombination of electrons and holes is minimized during photocatalytic reactions. The photocatalytic degradation of SMX was promoted by the attack of •OH, as evidenced by the detection of 2.2 μM •OH, using coumarin as a probe. This study highlights the potential application of MXene-derived oxide nanoheterostructures in wastewater treatment.