This study examined the social impact of the COVID-19 outbreak on Bangkok slum residents and the initiatives of Civil Society Organisations (CSOs) to relieve negative impacts. A mixed-methods study was conducted based on the Social Impact framework. In June 2020, a cross-sectional survey was carried out among 900 participants from nine slums in different zones of Bangkok. In July 2020, semi-structured interviews were conducted with 19 slum residents and four CSOs to gain in-depth information on the social impact of COVID-19 and CSOs' response. Out of 900 participants, 25.9% lost their jobs during the lockdown and 52.7% lost their income. The job and income loss increased the poverty rate within the participants from 51.6% to 91.7%. Participants limited their mobility and social activities during the lockdown. Stress was increased among 42.6% of all participants and the increased stress was associated with both income loss and self-quarantine. Due to financial constraints, a significant proportion of participants had to limit their food consumption and/or their consumption of nutritious but more expensive food. Almost one-tenth of the participants relied on donated food only. The majority of the participants (61.1%) could not access the income compensation scheme. COVID-19 forced Bangkok slums residents to live below the subsistence level in multiple ways with limited access to social protections. CSOs played an important role in relieving the suffering by providing food, survival kits, jobs, and access to COVID-19 test. Their agility, skills and knowledge about slums, and social capital enabled a rapid response to the crisis. Experienced local CSOs should be engaged as a bridge between urban slums and social protections. A holistic approach to combatting the COVID-19 crisis should be implemented. It is important to find the balance between preventing death from the virus and preventing suffering and death from an economic crisis.