<p>Rainfall variability and increasing grazing pressure pose growing vulnerability to grazing systems in semi-arid rangelands. This study develops and applies a Rainfall–Livestock Vulnerability Index (RLVI) to assess spatial patterns of grazing vulnerability and guide adaptive management in the Usangu catchment, Tanzania. The RLVI integrates climatic exposure (coefficient of variation, CV; standardized anomaly index, SAI), sensitivity (precipitation concentration index, PCI; length of growing season, LGS; rainy days, RD; and dry spells, ISD), and adaptive capacity represented by livestock density (LD) to produce a composite ward-level assessment. Results indicate moderate exposure (scores 1–2), reflecting stable annual rainfall (CV &lt; 20%) and near-normal anomalies (SAI − 0.11 to 0.01). Sensitivity is moderate to high (1.75–2.5), driven by moderately LGS (124–185 days), and RD (71–95 rainy days), seasonally concentrated rainfall (PCI 18–22), and recurrent ISD (3–5 days) indicating grazing systems that are generally functional but responsive to rainfall irregularities. Adaptive capacity reveals spatial contrasts controlled by LD. Wards with low LD (0.3–0.8 TLU ha⁻¹) exhibit high adaptive capacity (scores 3–4), whereas wards with high LD (&gt; 1.5 TLU ha⁻¹) show low adaptive capacity (scores 1–2). RLVI values range from 0.8 to 4.0, corresponding to low and moderate vulnerability classes. Overall, the RLVI demonstrates that grazing vulnerability emerges from interactions between climatic variability and livestock management, with stocking density as the dominant moderating factor. The framework provides a practical, transferable tool for climate-resilient grazing management in semi-arid rangelands. Interventions should prioritize wards within the moderate vulnerability category through seasonal destocking, rotational grazing, pasture resting, protection of mobility corridors, establishment of fodder reserves, and improved livestock water access.</p> Graphical Abstract <p></p> <p>This study illustrates the framework for assessing livestock vulnerability to rainfall variability in the Usangu Catchment, southern Tanzania. Multiple rainfall indicators including onset, cessation, total rainfall amount, Coefficient of Variation (CV), Precipitation Concentration Index (PCI), Standardized Rainfall Anomaly Index (SAI), Rainy Days (RD), Intra-Seasonal Dry Spells (ISDS), and Length of the Growing Season (LGS) were initially analysed. Pearson correlation and Variance Inflation Factor (VIF) tests identified six robust, non-redundant variables for developing the Rainfall–Livestock Vulnerability Index (RLVI), which integrates rainfall characteristics with livestock density (LD). The RLVI captures three components of vulnerability: exposure (CV, SAI), sensitivity (PCI, ISDS, RD, LGS), and adaptive capacity (inverse of livestock density). Spatial mapping at ward level revealed that vulnerability rises where high rainfall variability coincides with high livestock density. Lowland areas characterized by relatively short (&lt; 80 rainy days) and concentrated rainfall (PCI &gt; 20) with SD &gt; 1.0 TLU ha⁻¹) emerged as hotspots (RLVI &gt; 3.25). The findings highlight the need for adaptive management strategies such as flexible grazing calendars, forage reserves, and water-harvesting interventions to mitigate climate-induced vulnerability and sustain pastoral livelihoods in semi-arid ecosystems.</p>