According to the Islamic culture and civilization, tourism is deeply linked to Islamic thought and belief. It is one of the most profitable industries in the international fields and an important way for producing income and getting out of economic challenges of the local, national, regional, and international situations. Because the holy shrine of Imam Reza (AS) is placed in Mashhad, it has a substantial capacity to attract religious tourists; yet, in recent decades, the number of pilgrims and tourists has increased, but their average stay in the city has decreased. Accordingly, the main purpose of this study is to identify the factors affecting the attraction and retention of religious tourists in Mashhad metropolis. In order to achieve this goal, the qualitative research method and grounded theory of Glaser’ s approach have been employed. In this regard, the opinion of experts on the effective factors on attracting and retaining religious tourists in Mashhad metropolis were collected by semi-structured interviews. Through grounded method, they were categorized into open, selective and theoretical codes inferred from the identified concepts. The results of twelve interviews and reviewing and interpreting the data, led the researchers to 37 initial identifiers in the form of twelve sub-categories in five main categories including the characteristics of the destinations of the tourists and host community, tourist destination management, the characteristics and needs of pilgrims or religious tourists and the features of their travelling. Finally, the model of factors affecting the attraction and retention of religious tourists in Mashhad metropolis was presented, and the relations among categories were explained.