The increase in annual usage of plastics for different purposes has led to an increase in microplastics (MPs) particles in various environments including inside of buildings. In the present study, levels and characteristics of MPs in the indoor dust of buildings with different uses including residential house, mosque, hospital, kindergarten, and university in two cities, Bushehr port and Shiraz in Iran, were determined. Thirty dust samples from various buildings were collected. The average numbers of MPs in the indoor dust of buildings of Shiraz and Bushehr were 90.8 and 80.8 items/mg, respectively. Fiber and polyethylene (PE) were the most common shape and polymer types of identified MPs, respectively. The highest number of MPs was observed in the indoor dust of kindergartens with a mean number of 121 items/mg in Bushehr and 104 items/mg in Shiraz. Except for mosques, a significant difference (p-value <0.05) were not observed between the MPs levels of buildings with similar applications in Bushehr and Shiraz cities. In addition, the high daily intake to MPs via indoor dust ingestion and inhalation were found for infants in kindergartens and mosques. The results of this study showed that MPs are present in high concentrations in indoor environments and may pose a high exposure risk for different age groups.