Background: Routine immunization is one of the most effective public health interventions for preventing vaccine-preventable diseases and reducing childhood morbidity and mortality. However, inadequate maternal knowledge remains a major barrier to optimal immunization uptake and completion in Nigeria. Behaviour Change Communication (BCC) has been identified as a potentially effective strategy for improving maternal knowledge and promoting positive immunization behaviours. This study evaluated the effect of BCC on knowledge of routine immunization among women of childbearing age in Ebonyi State, Nigeria. Methodology: A quasi-experimental study involving intervention and control groups was conducted among 910 women of childbearing age attending antenatal, postnatal, and immunization clinics in selected health facilities in Ebonyi State. Participants in the intervention group received a structured three-month BCC package comprising health education sessions, posters, charts, audio-visual materials, and educational flyers, while the control group received routine health education. Data were collected at baseline and post-intervention using a structured questionnaire. Descriptive statistics, Chi-square tests, and Odds Ratios (ORs) with 95% confidence intervals (CIs) were used for data analysis at a significance level of p < 0.05. Results: Baseline knowledge was poor and comparable between the intervention and control groups (5.4% vs. 6.9%; χ² = 3.02, p = 0.083). Following the intervention, adequate knowledge increased significantly from 5.4% to 74.4% among women in the intervention group (OR = 50.40; 95% CI: 31.67–80.21; p = 0.001), whereas only a marginal increase was observed in the control group (6.9% to 7.9%; p = 0.590). Post-intervention knowledge was significantly higher among participants exposed to BCC than controls (74.4% vs. 7.9%; p < 0.001). Conclusion: Behaviour Change Communication significantly improved maternal knowledge of routine immunization. Integrating structured BCC strategies into routine maternal and child health services may enhance maternal immunization literacy and improve vaccination outcomes.