nformal settlements remain a major urban development challenge in Nigeria due to rapid urbanisation, poverty, high housing costs, weak planning control, insecure land tenure, and inadequate infrastructure provision. These settlements are often characterized by poor sanitation, inadequate drainage, flooding, poor housing quality, overcrowding, weak road access, and limited access to basic services. This study examined the roles of architects, quantity surveyors, town planners, and project managers in the sustainable upgrading of informal settlements in Nigeria. A quantitative descriptive survey design was adopted, and data were collected from 427 built environment professionals comprising architects, quantity surveyors, town planners, project managers, and other related professionals in Nigeria. Data were analysed using frequency, percentage, mean, standard deviation, ranking, and Cronbach’s Alpha reliability test. The findings showed that the major causes of informal settlement growth were high cost of formal housing, poverty, rapid urbanisation, weak planning control, and limited access to housing finance. The leading sustainability challenges were poor sanitation, inadequate drainage, flooding, poor housing quality, and waste management problems. The study further found that architects contribute through affordable and climate-responsive housing design; quantity surveyors through cost planning, lifecycle costing, and cost control; town planners through spatial planning, reblocking, and citywide integration; and project managers through stakeholder coordination, risk management, scheduling, and implementation control. The study concludes that sustainable upgrading of informal settlements in Nigeria requires an integrated professional framework supported by community participation, institutional coordination, adequate funding, and postproject monitoring.