The construction industry is recognized as one of the industries most exposed to climatic conditions. Construction projects are mainly executed in an outdoor environment and the activities are considered weather-sensitive. Severe weather conditions can directly affect the productivity and efficient operation of construction projects. In addition, the weather is known to be one of the main factors that decrease labour productivity in construction projects causing project delays, cost overruns, and contractual claims between contractors and clients. Oman has a hot climate with very high temperatures in summer, warm winters, and low annual rainfall. During extremely hot weather, labour productivity significantly decreases, as construction work may stop partially or completely. Therefore, this paper investigates and quantifies the hot and humid weather effects in construction projects in Oman. A construction productivity model was developed using the work/rest schedule proposed by the National Institute for Occupational Safety and Health (NIOSH), USA. The daily weather temperature and relative humidity of Muscat were inputs into the model, and the expected productivity in terms of working hours was the output of the model. The model was applied to case studies, which involved three completed construction projects under different testing scenarios in Muscat. Results indicate that implementing the influence of hot and humid weather can lead to an extension of 3–38% longer project duration compared to the planned duration.