The solution to the problem of managing the cost of an investment and construction project under conditions of risk and uncertainty is presented in the issue. The external and internal factors of project risks are analyzed. The concept of uncertainty is characterized as a property of the external and internal environment of the project, which underlies the occurrence of risks. Additional prerequisites for the emergence of risks, both objective and subjective, are considered. An authors’ model for managing the cost of an investment and construction project based on a process approach has been developed based on the above mentioned prerequisites. The content of the stages of the project cost management model has been determined. The possibility of managing the cost of the project through the analysis of budget spending has been investigated. In the course of the study, a comparison was made between the work schedule in accordance with the basic project plan and the actual work schedule with deviations from the project plan. Unfavorable deviations of the baseline indicators of the project were revealed, both in terms of work execution time and in the cost of work performed. It has been proved that the adverse impact of risk factors on the overall performance of the project, expressed by indicators of investment efficiency, is reduced as a result of the application of the process model for managing the cost of an investment and construction project.