Prior research found that testosterone change after defeat predicted the decision to compete against the same opponent, but testosterone change after victory was unrelated to competitive behavior. The present research tested whether testosterone responses have differential effects on competitive decision-making depending on whether an individual either barely or decisively won a competition. Seventy-one undergraduate males provided an afternoon saliva sample and then participated in a laboratory cognitive contest in which they were randomly assigned to experience a relatively close or decisive victory against a male confederate. Participants provided a second saliva sample after the competition and then chose whether to: (a) compete against the same opponent, (b) compete against a new opponent, or (c) complete an alternative non-competitive task. Participants also reported how much they enjoyed the competitive task. Testosterone change and the propensity to compete were positively related after a decisive victory, but were negatively related after a close victory. These effects were driven by the decision to compete against a new opponent. In fact, very few participants chose to compete against the same opponent. Testosterone change after a decisive victory was also positively associated with participants' self-reported enjoyment of the competitive task. Together, these results provide new evidence that a close versus decisive victory moderates the effect of testosterone change on future competitive behavior, an effect that may be linked to changes in reward processing systems.