Background: People with serious mental illnesses (SMI) have higher levels of loneliness than the general population. Furthermore, people with SMI tend to be less satisfied with their housing and tend to move more frequently. Aim: This study aims to examine relationships between housing variables (whom they live with, duration of residence, and satisfaction) and loneliness among individuals with SMI. Methods: Data were collected from 188 adults with SMI in greater Philadelphia area. Classification and Regression Trees (CART) were used to examine whether whom they live with, duration of residence, and housing satisfaction were associated with loneliness. Results: Housing satisfaction was found to be the most prominent predictor of loneliness. Those who were unsatisfied with their overall housing conditions always had the highest level of loneliness, regardless of other factors. Even if they were satisfied with their housing conditions, their loneliness was higher if they had just moved to the new residence. Participants had lower loneliness the longer they lived in a residence and had the lowest loneliness levels after about three years. Conclusion: Housing is associated with loneliness among people with SMI. Psychiatric service providers should increase support to factors contributing to housing satisfaction and duration of residence, including active engagement in the community.