Reactive oxygen species (ROS) are central regulators of plant growth and stress responses, and their cellular levels are tightly controlled by antioxidant systems, including the evolutionarily conserved catalases that decompose hydrogen peroxide (H2O2) predominantly within peroxisomes and glyoxysomes. Despite their importance, key aspects of catalase biogenesis, regulation, subcellular targeting, and potential extra-peroxisomal functions remain poorly understood. Using affinity purification of the UV-B photoreceptor UVR8 coupled with mass spectrometry, we identified a REGULATOR OF CHROMATIN CONDENSATION 1–like protein in Arabidopsis, which we named CATALASE-INTERACTING RCC1-LIKE 1 (CAIR1). CAIR1 interacts with all three catalase isoforms (CAT1–CAT3) as well as their chaperone NO CATALASE ACTIVITY 1 (NCA1). Loss-of-function cair1 mutants partially phenocopy cat2 and nca1, exhibiting reduced catalase activity, enhanced sensitivity to oxidative stress and alkaline growth conditions, and impaired primary root elongation. Mechanistically, cytosolic interaction between CAIR1 and CAT2 enhances total cellular catalase activity by facilitating peroxisomal import and proper subcellular localization of CAT2. In the absence of CAIR1, CAT2 forms aggregates, likely accounting for the observed loss of catalase activity. Notably, CAIR1 undergoes reversible, redox-dependent oligomerization that strengthens its interaction with catalases. Mutation of CAIR1 at Cys-356 and Cys-545 compromises this interaction under elevated ROS conditions and fails to rescue the oxidative stress sensitivity of cair1 mutants. Moreover, UV-B exposure suppresses catalase activity by weakening the interaction between CAIR1 and catalases, linking environmental light signaling to cellular redox regulation. Together, our findings reveal CAIR1 as a dynamic redox-responsive regulator of catalase activity that maintains cellular redox homeostasis by coordinating catalase localization and function through reversible oligomerization.