Background <p>Expanding free maternal health services in Ethiopia advances gender justice. However, structural inequities, low satisfaction, and limited use of labour companionship remain. Lower self-efficacy in companions and views of labour wards as female-only spaces are major barriers. This study measured the effectiveness and cost-effectiveness of a structured Health Belief Model (HBM)-based education session for chosen labour companions in public facilities.</p> Methods <p>This parallel-group cluster-randomized trial was conducted in 12 public health facilities in Addis Ababa from January 15 to June 5, 2025 implementation phase. Facilities were randomized 1:1, assigning six clusters (222 primigravidae dyads) to each arm. Primigravidae were screened at 12–14 weeks of gestation. The intervention group completed five HBM-based sessions for labour companions from 26 weeks to admission for labour and delivery. The control group received only standard antenatal care. The primary outcome was maternal satisfaction, measured by the 25-item Pregnancy and Childbirth Questionnaire (PCQ). Secondary outcomes included delivery mode, labour duration, and newborn complications. We used Generalized Estimating Equations (GEE) for estimation and validated the results using a 9,999-replication Wild Cluster Bootstrap (WCB). Economic assessment took a health system view, using micro-costing to calculate the Incremental Cost-Effectiveness Ratio (ICER) per newborn complication avoided.</p> Results <p>The intervention had a highly significant positive impact on enhancing maternal satisfaction (β = 21.14, WCB 95% CI: 14.14 to 28.50, <i>p</i> &lt; 0.001) compared to the control group. A notable reduction in prolonged labour &gt; 8&#xa0;h (aOR = 0.82 =, 95% CIWCB: [0.74, 0.91], PWCB &lt; 0.001) was accompanied by a significant drop in newborn complications (4.5% vs. 11.26%; aOR = 0.93, 95% CIWCB: [0.90, 0.97], PWCB = 0.0041). Descriptively, higher spontaneous vaginal delivery rates were observed in the intervention group (77.0% vs. 66.2%), alongside reduced Cesarean pathways (14.9% vs. 23.0%), though these modal variations did not achieve formal statistical significance (<i>p</i> &gt; 0.05). Economic modeling revealed that the intervention was highly cost-effective, demonstrating a &gt; 90% probability of cost-effectiveness at Ethiopia’s willingness-to-pay benchmark, driven by the avoidance of newborn complications and neonatal intensive care costs.</p> Conclusions <p>Preparing labour companions through a socio-culturally adaptive, theory-based approach substantially improves subjective maternal satisfaction and optimizes key physiological parameters, including labour duration and immediate neonatal transition outcomes. While it does not significantly alter the mechanical mode of birth or maternal complications, it provides an exceptionally cost-effective strategy that optimizes low-resource health systems, offering high clinical value without escalating institutional overhead costs.</p> Trial registration <p>It was registered with Pan African Clinical Trials Registry (PACTR) with Trial ID: PACTR202503666794479 on March 17, 2025, retrospectively registered due to a lack of awareness. URL: <a href="https://pactr.samrc.ac.za/TrialDisplay.aspx?TrialID=33509">https://pactr.samrc.ac.za/TrialDisplay.aspx?TrialID=33509</a></p>