Urban pollution is increasing at an alarming rate within the catchments of forested riverine systems in sub-Saharan Africa, Nigeria inclusive. Assessing the impact of pollution in riverine systems in the Niger Delta region is still within the use of physico-chemical variables and biota-based assemblage. In covering this important gap in freshwater biomonitoring, we developed a macroinvertebrate-based multimetric index (MMI) that would be useful in monitoring, assessing, and managing forested riverine sites affected by urban pollution. We collected macroinvertebrates and physico-chemical samples monthly at 20 sites in 11 streams. Physico-chemical variables were analysed using standard methods while a kick sampling procedure was employed in collecting macroinvertebrates. The physico-chemical variables were used to classify the sites into three disturbance categories: least-impacted sites (LIS), moderately impacted sites (MIS), and heavily impacted sites (HIS). Fifty-nine candidate macroinvertebrate metrics were selected and screened for developing our MMI. We employed sensitivity, seasonality, repeatability and redundancy tests, and metric scoring in screening and arriving at the final metrics for the MMI development. Five metrics were finally selected for the MMI development: Trichoptera abundance, %Chironomidae+Oligochaeta, Coleoptera richness, Simpson diversity, and Shannon–Wiener index. Correlation in the selected metrics with physico-chemical variables showed that Simpson diversity was negatively correlated with pH in the MIS and Coleoptera richness was positively correlated with dissolved oxygen (DO) and water depth in the LIS. Nitrate, biochemical oxygen demand (BOD), conductivity, and water temperature were negatively correlated with %Chironomidae+Oligochaeta in the HIS. This MMI can aid river and stream managers in assessing the ecological conditions of rivers and streams in the Niger Delta region of Nigeria.