Objective: To examine the perceptions of socially just fares among urban public bus users in Portoviejo and to assess the potential impacts of fare increases on the most vulnerable populations and the subsequent changes in their mobility behavior. Method: To unpack the multiple facets that shape the notion of justice, we conducted a two-stage clustered observational study using a survey administered to a sample of urban bus users. The data collected were scrutinized using three analytical methods: exploratory description, hypothesis testing, and binary logistic regression. Results: Our findings revealed that most bus users are women, under 35 years of age, and earn at most the minimum wage. These younger, lower-income users who utilize the bus less frequently, tend to view lower fares as just compared to their counterparts in different user profiles. When fares exceed what they perceive as just, these users tend to mitigate their bus usage, opting to walk more or invest in a bicycle or motorcycle for their daily transport needs. Conclusion: The intersection of public transport fares and social justice is not an extensively researched area, and one key theoretical challenge is balancing the principle of universal provision with users' varying payment abilities under the framework of social justice. Future research on this intersection should explore factors beyond fares, such as safety, accessibility, territorial coverage, and travel time, and consider alternative fare models to address socioeconomic disparities and improve overall well-being.