Despite the prominence of fitness testing in school physical education (PE), there is a sparsity of research examining the antecedents of students’ affective experiences during fitness testing lessons. This study aimed to investigate the associations among task- and ego-involving motivational climates, perceived physical competence, physical performance, enjoyment, and anxiety during two different types of PE fitness testing lessons. Altogether, 645 Finnish students from Grade 5 (50% boys, Mage = 11.2, SD = 0.36) and Grade 8 (47% boys, Mage = 14.2, SD = 0.35) participated in two fitness testing lessons with different content (lesson 1: 20-meter shuttle run test and a test of flexibility; lesson 2: curl-ups, push-ups, 5-leaps, and a catching-throwing combination test). Students’ experiences were collected using short questionnaires immediately after the lessons. Structural equation modeling was applied to examine the direct and indirect associations among study variables. Results indicated that task-involving climate and perceived competence increased students’ enjoyment and decreased their anxiety levels whereas ego-involving climate had no effect on students’ enjoyment but increased their anxiety levels. In addition, students’ actual physical performance as a mediator between motivational climate and affects, or as a direct predictor of affects, was limited. Strategies advancing task-involving motivational climate and students’ perception of competence should be employed to increase enjoyment and decrease anxiety during PE fitness testing lessons.