The growing prevalence of various heavy metals and organic pollutants, such as dyes, pharmaceuticals, and agrochemicals, in wastewater poses significant environmental and public health concerns, necessitating the development of green, innovative, low-cost, and highly effective treatment technologies. Conventional methods often fall short in effectively eliminating these pollutants, prompting the exploration of advanced materials and techniques. This chapter examines recent advances in wastewater treatment using NiO-based nanocomposites, focusing on their enhanced adsorptive and photocatalytic capabilities. The chapter examines the underlying adsorption mechanisms, emphasizing the critical role of operational factors like pH, adsorbent dosage, and contact time in maximizing pollutants’ removal efficiencies. Furthermore, the chapter explores the utilization of NiO-based nanocomposites as highly effective photocatalysts, leveraging the NiO’s light-harvesting abilities and charge separation dynamics to facilitate organic pollutants’ degradation. The synergistic effects observed when NiO is combined with complementary nanomaterials, such as carbon-based structures, semiconductors, and other metal oxides, to generate hybrid nanocomposites have been shown to significantly enhance both the adsorption and photocatalytic activity. The chapter also addresses the challenges associated with the practical implementation of NiO-based nanocomposites, such as recyclability and stability, while discussing emerging strategies and future research directions to overcome these barriers. The comprehensive insights offered in this chapter help to further the ongoing efforts to develop effective, sustainable, and scalable solutions for removing a wide range of pollutants from wastewater, paving the way for better environmental remediation and water resource management.