Multi-parent Advanced Generation Inter-Cross (MAGIC) populations offer high recombination and genetic diversity, providing opportunities for dissecting complex agronomic traits. In this study, 230 MAGIC rice recombinant inbred lines derived from eight indica founders were evaluated at the Assam Rice Research Institute, AAU, Titabor, to assess variation in five key traits: grain weight, grain number per plant, grain length, grain breadth, and 100-grain weight. Analysis of variance (ANOVA) revealed significant genotypic variation (p < 0.001) for grain yield per plant and grain number per plant, while grain size traits showed limited differentiation among genotypes. These traits exhibited high genotypic and phenotypic coefficients of variation (> 50%) and high broad-sense heritability (> 89%), indicating a substantial genetic contribution to phenotypic variation under the experimental conditions. Grain yield per plant showed a strong positive correlation with grain number per plant (r = 0.97), identifying grain number as the primary determinant of yield. Multivariate analyses, including principal component analysis and hierarchical clustering, revealed structured phenotypic variation within the population. A Composite Trait Index (CTI) facilitated the identification of superior lines, with Plots 18, 70, and 85 emerging as promising genotypes. Collectively, these findings demonstrate the phenotypic diversity and breeding potential of the MAGIC population for improving rice yield under rainfed lowland conditions of Northeast India.