Background: Although it is well established that students' adaptive reactions towards errors promote learning outcomes, little is still known about the role of error feedback in promoting these reactions. Aim: Through a targeted intervention based on an online teaching unit, this study aimed at testing whether supportive error feedback promotes more adaptive students' reactions towards errors and higher learning outcomes. Sample: A total of 250 (M[subscript age] = 12.18, SD = 0.89; 46.4% girls) Italian middle school students took part in the intervention. Students were randomly assigned to either a discouraging error feedback condition (n = 124) or a supportive error feedback condition (n = 126). Method: The intervention consisted of an online teaching unit, which students filled in at home, that was divided into pre-test, intervention and post-test phases. During the intervention, students replied to training questions and every time they made an error, informative feedback appeared: supportive smileys and sentences in the supportive feedback condition, and disappointed smileys and sentences in the discouraging feedback condition. Before the intervention, students filled in the pre-test and after the intervention, students reported their reactions towards errors and filled in the post-test. Results: Receiving supportive feedback resulted in more adaptive affective-motivational reactions towards errors, which in turn were related to more adaptive action reactions towards errors. Differently from our expectations, action reactions towards errors were not related to the post-test scores. Conclusions: Our findings can inform the development of online teaching units that promote an error-oriented approach.