Background: Childhood immunization remains one of the most effective public health interventions for preventing vaccine-preventable diseases and reducing under-five mortality. Although India's Universal Immunization Programme has substantially improved vaccination coverage, incomplete immunization continues to be a significant public health concern, particularly in resource-constrained settings. Identification of factors associated with incomplete vaccination is essential for developing targeted interventions to improve immunization coverage. Objectives: To assess the immunization coverage among children aged 12–23 months attending a tertiary care hospital in Rajasthan and to identify factors associated with incomplete vaccination. Methods: A hospital-based cross-sectional analytical study was conducted from January 2025 to June 2025 among 380 children aged 12–23 months attending the Pediatric Outpatient Department and Immunization Clinic of a tertiary care hospital in Rajasthan. Participants were selected using systematic random sampling. Data were collected using a pretested structured questionnaire, and immunization status was verified primarily through the Mother and Child Protection (MCP) card or immunization card. Descriptive statistics were used to summarize the data. Associations between independent variables and immunization status were assessed using the Chi-square test or Fisher's exact test. Variables significant in bivariate analysis were included in multivariable logistic regression. A p-value <0.05 was considered statistically significant. Results: Among the 380 enrolled children, 302 (79.5%) were fully immunized and 78 (20.5%) were incompletely immunized. Rural residence, maternal education below the secondary level, home delivery, fewer than four antenatal care visits, and absence of an MCP/immunization card were significantly associated with incomplete immunization. Multivariable logistic regression identified absence of an MCP/immunization card, home delivery, lower maternal education, inadequate antenatal care, and rural residence as independent predictors of incomplete vaccination. The most commonly reported reasons for incomplete immunization were caregiver forgetfulness, child illness on the scheduled vaccination day, inadequate awareness regarding subsequent vaccine doses, and difficulty accessing vaccination services. Conclusion: Although immunization coverage among children attending the tertiary care hospital was encouraging, a substantial proportion of children remained incompletely vaccinated. Maternal education, utilization of maternal healthcare services, documentation of immunization, and place of residence significantly influenced vaccination completion. Strengthening caregiver counselling, promoting institutional deliveries, improving antenatal education, implementing reminder systems, and enhancing outreach services in rural areas may improve vaccine completion and contribute to achieving universal immunization coverage