Social housing providers in many advanced economies have been directed towards market-orientated and commercial business models, whereas the scope of the private rental sector has been increasingly expanded to the provision of housing for low-income households. Where these developments coincided, the demarcation between the activities of social and private landlords has blurred, with the result of increasing competitive pressure on both groups. This paper sheds light on the behavioural aspects of competition by introducing the concept of inter-landlord rivalry in local rental housing markets. Drawing on data from 36 in-depth interviews with social and private landlords in Coventry/England and Breda/the Netherlands, this study shows that existing perceptions of rivalry among most landlords are exceptional in low-income renting but pronounced in more expensive, commercial rental segments. The paper will demonstrate that these perceptions are highly subjective and non-reciprocal. Moreover, competitive perceptions and interactions are strongly affected by political and market structural settings, and appear to be dependent on the trade-off between the social mission and commercial goals of individual housing associations in the social housing sector.