Decisions about one's health are often accompanied by uncertain outcomes, which may be either positively or negatively valenced. The presence of this uncertainty, which can range along a continuum from risk to ambiguity (i.e., decisions in which the outcome probabilities are known or unknown), can be perceived as threatening, and individuals tend to be averse to uncertain outcomes, and will attempt to avoid uncertainty when possible. We proposed that one way to reduce uncertainty aversion could be to provide opportunities to affirm one's core values, or "self-affirmation." Prior research has suggested that self-affirmation promotes health behaviour by providing a buffer against potential threats to the self. However, the degree to which self-affirmation affects decision-making is still unclear. Across two studies, we tested the effects of a self-affirmation manipulation on risk (Study 1) and ambiguity (Study 2) preferences for both potential gains and losses. In both studies, we found that, compared to the non-affirmed group, affirmed individuals were more accepting of uncertainty when the decision involved potential gains, but not for potential losses. Further, for risky decisions, the increased acceptance of uncertainty came at the expense of making choices consistent with expected value, such that self-affirmed individuals made more disadvantageous choices than non-affirmed individuals. Our results suggest both benefits and costs of self-affirmation in the context of risky choice, an important finding given the many applications of self-affirmation in behavioral decision making contexts.