De-biasing governors' decisions is crucial. However, this debiasing requires a complete understanding of the biases. Optimism bias is a type of unconscious cognitive bias. It refers to an unrealistically favorable attitude that people have towards themselves and people who are close to them. Optimism bias is defined as the difference between a person's expectation and the outcome that follows. Accordingly, this study, by examining optimism in Islamic governance decisions, answers the question of what the mechanisms and contextual factors of optimism bias in Muslim governors' decisions are and what their outcomes are. Therefore, with a comparative approach and a realist systematic review method of optimism bias in two valuable Islamic sources in the field of governance, namely the Holy Quran and Nahjul Balagha, it was concluded that optimism bias provides the basis for disruption in the decisions of rulers, both in the field of theoretical reason and in the field of practical reason. Therefore, the mechanisms of this bias are not limited to a lack of awareness, but rather a lack of implementation of Islamic teachings, which has a significant impact on the emergence of optimism bias, which is realized in the behavior of governors in governance justice, which can be recognized by the people.