In China, studies on the regional risk assessment of hazardous chemicals have been carried out for only a few years, and there are few studies on hazardous chemicals leaking into seas. Previous regional-risk-assessment methods considered a single risk factor for most assessment targets, and comprehensive considerations of risk sources and sensitive resources for a study area are not sufficiently included. Based on previous work, this study established a regional-risk-assessment method for hazardous chemicals leaking into seas. This method considered the hazards of hazardous chemicals and the tolerance of the regional environment by means of a case study in Tianjin. The results showed that the risk level of the enterprise was Grade I, classified as a high-risk source of hazardous chemicals; the main reasons were the strong toxicity and large quantity of hazardous chemicals. This method provides technical support for scientifically assessing marine-environmental-risk levels for hazardous-chemical-leakage areas and for carrying out risk-prevention and restoration assessments of hazardous chemicals leaking into seas.