Rarefaction in capillary density is a hallmark of hypertension-mediated microvascular damage. This study aimed to assess the association between clinically accessible inflammatory markers, including the systemic immune-inflammation index (SII), and retinal capillary density, as well as other indicators of microvascular damage. We conducted a cross-sectional analysis of data from 132 consecutive patients with established primary hypertension at the Royal Perth Hospital's tertiary hypertension clinic. All patients underwent noninvasive optical coherence tomographic angiography (OCT-A) for the assessment of retinal capillary density in the foveal region (CDF) and blood sampling for inflammatory markers. We examined the association of the SII - calculated as the product of the neutrophil-lymphocyte ratio and platelet count - and its individual components with retinal capillary rarefaction and other markers of microvascular damage, such as the urinary albumin-creatinine ratio (UACR) and estimated glomerular filtration rate (eGFR). The results were adjusted for relevant co-variates, including age, sex, 24-h SBP, BMI, antihypertensive medications, lipid levels, and diabetes status. Retinal capillary rarefaction was associated with increased white cell count, particularly neutrophils, and the SII. Through predictive margin analysis, an optimal cut-off value of 600 x 109/l for SII was determined for median CDF of 34.1 mm2. The analysis showed a reduction in CDF of 1.3 mm2 for every 250 x 109/l increase in SII. Additionally, higher SII levels (≥ 600 x 109/l) were associated with elevated high-sensitivity C-reactive protein (hs-CRP) levels and markers of microvascular damage, such as increased UACR and reduced eGFR. In patients with primary hypertension, SII and related inflammatory markers were associated with retinal rarefaction and renal indices of microvascular damage. SII may serve as a useful clinical marker of microvascular damage in the retinal and renal vascular bed.