Evidence for the protective role of dispositional mindfulness for athletic performance, stress, and mood among elite athletes has been demonstrated through correlational and interventional studies. The effects of state mindfulness on athletic functioning in day-today training contexts remains unclear. We examined the effects of state mindfulness on mood, biological markers of stress, and self-rated athletic performance in elite athletes during daily training. We used a diary study design to collect data on state mindfulness, mood, self-rated athletic performance, and salivary cortisol directly following training sessions of 78 elite athletes. For each athlete, a total of 27 data points were obtained across 9 weeks with data collected on a separate day, 3 days per week. Data were analyzed with multilevel structural equation modeling. At both the between-person and within-person levels, state mindfulness was significantly and negatively related to total mood disturbance and maladaptive dimensions of mood, including anger, confusion, depression, fatigue, and tension. Conversely, state mindfulness was positively related to vigor and self-rated athletic performance. Relations between state mindfulness and biological markers of stress were nonsignificant. Overall, findings of the present study provide preliminary empirical evidence supporting the utility of mindfulness interventions for improving state mindfulness of elite athletes. Such interventions may increase the positive mood of athletes and their performance during training.