Many studies have shown that test anxiety is negatively related to achievement, but studies testing reciprocal relations with achievement are largely lacking. Furthermore, nonadaptive forms of emotion regulation (ER) have been linked to elevated anxiety but there are few studies of ER and test anxiety specifically, and ER is rarely considered alongside relations between test anxiety and achievement. Given that strategies to regulate emotions about exams could have a substantive bearing on test anxiety, we aimed to gain a more comprehensive understanding of how test anxiety relates to achievement by also considering ER strategies. Data were collected from 533 students (Mage = 16.4 years, male = 25.8%) in a sample of upper secondary students over five waves with 6-month intervals. Achievement was measured at the first, third, and fifth waves, and test anxiety and ER, at the second and fourth waves. Data were analyzed in a series of structural equation models to model relations between test anxiety and achievement over time and test anxiety and ER contemporaneously. Test anxiety showed negative reciprocal relations with achievement and positive reciprocal relations with two ER strategies, catastrophizing and rumination. Of the remaining ER strategies, test anxiety followed putting into perspective in a negative unidirectional relation; positive reappraisal and refocus on planning followed test anxiety in negative unidirectional relations; and acceptance, self-blame, and other blame followed test anxiety in positive unidirectional relations. Catastrophizing, rumination, and putting into perspective may be useful foci for test anxiety intervention.