<p>Rapid urbanisation in developing nations has led to the proliferation of informal settlements with deteriorating environmental conditions; yet comprehensive assessments of environmental quality dynamics in coastal slum settlements remain limited. This study examines factors influencing urban environmental quality in five coastal slum settlements (Bariga, Oworoshoki, Majidun, Iwaya, and Ijora-Badia) in Lagos Metropolis, Nigeria, addressing the knowledge gap in understanding environmental quality-wellbeing relationships in informal urban areas. An integrated spatiotemporal approach combining Principal Component Analysis, Geographic Information Systems-based Multi-Criteria Decision Analysis, Analytical Hierarchy Process, and Weighted Linear Combination was employed. Primary data were collected through structured questionnaires from 250 households, in-depth interviews, and field observations, while secondary data included Landsat 8 satellite imagery and census data. Principal component analysis revealed duration of stay, water provision, community development, and building characteristics as primary factors explaining 12.3–30.8% of total variance across settlements. The spatial analysis demonstrated significant environmental quality variations, with 61–80% of areas experiencing high environmental impact. Key findings indicate 67–85% of residents lack adequate recreational facilities, 76.5% have no street lighting, and 75% lack proper walkways. Open dumping constitutes the predominant waste disposal method (25-37.5%), while 34.5% rely on sachet water for drinking. The study concludes that environmental quality degradation in coastal slums results from inadequate infrastructure, poor governance, and limited community participation. Multi-stakeholder collaboration, policy reforms, and sustainable urban planning interventions are essential for improving environmental conditions and residents’ well-being in informal coastal settlements.</p>