This study examines the patterns, trends, and causal factors of internal migration in Sikkim using Census of India data for 2001 and 2011 (D3 Migration Tables) together with the Multiple Indicator Survey (MIS) 2020–21. The results show that migration in Sikkim increased from 1,86,987 persons in 2001 to 2,47,049 in 2011, with the migration rate rising from 34.57% to 40.46%, above the national average of 38%. Intra-district migration is the dominant stream, growing from 48.37% to 51.00% of all migrants, while inter-district migration's share declined slightly despite an absolute increase. Interstate in-migration is concentrated from West Bengal, Bihar, and Uttar Pradesh, while out-migration flows toward more economically developed states such as Delhi, Maharashtra, and Karnataka. Marriage is the principal reason for female migration across all districts, while work and employment dominate male migration, with East District and its capital, Gangtok, emerging as the state's primary migration hub owing to its administrative, educational, and economic importance. A district-wise composite index of selected urban-development indicators confirms East District's pre-eminence, followed by South, North, and West Districts. The findings highlight the need for balanced regional development to reduce migration-driving disparities across Sikkim's districts.