This study reports two independent meta-analyses on the relationship between (a) classroom climate and academic achievement; and (b) school climate and academic achievement. The analyses were based on extant correlational research studies at the K-12 level that were published between 2000 and 2020. The relationships were analyzed in terms of various moderator variables. The first dataset included 53 research studies focusing on classroom climate and academic achievement, and the second dataset included 37 research studies on school climate and academic achievement. The meta-analyses were carried out with a random effects model. Fisher's effect size was calculated for each study. Publication bias was tested with Egger's linear regression method and Duval and Tweedie's trim and fill method. We used the Q-test for moderation analyses and I2 test for heterogeneity analyses. The results revealed that the mean effect size for the relationship between classroom climate and academic achievement was small, while it was medium for the relationship between school climate and academic achievement. The effect sizes regarding classroom climate differed significantly only in terms of the academic area moderator variable, and the effect sizes regarding school climate differed in terms of school area, report type, and unit of analysis. This meta-analysis study confirmed that school climate and classroom climate are significant correlates of academic achievement, and the dataset in this study revealed a greater magnitude of relationship in favor of school climate (although a comparison is not meant in this study). This study suggests that policymakers and practitioners should invest their efforts in promoting sound school climates while maintaining an emphasis on classroom climate components, as the two types of climates are intertwined.