In healthcare settings, a climate that encourages speaking up among staff is believed to enhance patient safety and quality of care. However, the specific mechanisms of this relationship remain underexplored. Particularly, there is a need to understand how components of teamwork, such as situation monitoring, can be linked to the impact of a speaking-up climate on relevant outcomes. This study aimed to investigate the direct and indirect effects of a speaking-up climate on patient safety and quality of care using situation monitoring as a potential mediator. This cross-sectional study used survey data from 380 staff nurses who provided direct patient care at three Korean hospitals. Structural equation modeling was utilized to test a hypothesized mediation model using Mplus 7.0. Our data analysis confirmed the partial mediation model. As hypothesized, a speaking-up climate directly improved patient safety (β = 0.384, p < 0.001) and quality of care (β = 0.393, p < 0.001). Also, we found that indirect effects of a speaking-up climate on patient safety (β = 0.224, p < 0.001) and quality of care (β = 0.186, p = 0.005) through situation monitoring were significant. These results indicate that situation monitoring was found to significantly mediate the relationship between a speaking-up climate, patient safety, and quality of care. Our study demonstrates that the positive impact of a speaking-up climate extends beyond improving nurses' speaking up. Further, fostering a speaking-up climate can significantly improve patient safety and quality of care, and situation monitoring has a critical role in this relationship. These findings contribute to understanding how encouraging a speaking-up climate could benefit patient safety and care quality in healthcare organizations.