This paper examines whether organizational slack is associated with firms’ voluntary disclosure of corporate social responsibility (CSR), sustainability, and integrated reporting. This is an empirical research study using archival data based on a sample of public firms listed on the Korea Exchange from 2005 to 2016. We manually collected CSR reports, sustainability reports, and integrated reports (IRs) that were published during our sample period. We found that human resource slack was highly related to the publication of corporate social responsibility, sustainability, and integrated reports. Firms initiating such disclosure in their industry groups were likely to have slack in permanent employees as well as financial slack. Additionally, integrated reporting, which is a recent improvement in the delivery of financial and non-financial information, was positively associated with an excess number of regular employees. This study provides evidence that slacks in regular employees are related to a greater degree of voluntary disclosure via standalone CSR or sustainability reporting as well as integrated reporting. These findings suggest that slacks or excess human resources play a crucial role in voluntary corporate disclosure.