<p>Preservation of acetabular bone stock during primary total hip arthroplasty (THA) is increasingly important, particularly in younger patients with higher lifetime revision risk. Robotic-arm assisted THA has the potential to improve reaming precision and minimize unnecessary bone removal; however, comparative data using robust surrogate measures of acetabular bone preservation remain limited. This prospective observational cohort study included 101 consecutive primary THAs performed by a single surgeon between January 2024 and January 2025. Fifty hips underwent conventional manual THA (CTHA) and 51 hips underwent robotic-arm assisted THA (RTHA) using a CT-based robotic platform. Native femoral head diameter was measured intraoperatively and implanted acetabular cup size was recorded. Acetabular bone resection was assessed using linear (c–f), relative ((c–f)/f), and volumetric ((c³–f³)/f³) surrogate indices. Cup-to-head diameter ratio was analyzed as an independent measure of cup proportionality. The RTHA group demonstrated significantly smaller linear (1.63 ± 1.09 mm vs. 3.30 ± 1.45 mm), relative (0.03 ± 0.02 vs. 0.07 ± 0.03), and volumetric (0.10 ± 0.07 vs. 0.23 ± 0.11) indices of acetabular bone resection compared with the CTHA group (all p &lt; 0.001). The median cup-to-head ratio was significantly lower in RTHA (1.02 [IQR 1.02–1.04]) than CTHA (1.07 [IQR 1.06–1.09]; p &lt; 0.001), with a large effect size (r = 0.64). RTHA may be associated with greater preservation of native acetabular bone compared with CTHA, as reflected by reduced linear, relative, and volumetric surrogate indices of reaming. These findings indicate that CT-based planning with semi-autonomous haptic control enables more anatomically precise acetabular preparation with improved bone conservation.</p>