This study investigated the effects of Spirulina (Arthrospira plantensis) supplementation level, basal diet, sire breed, and sex on wool quality parameters in purebred and crossbred Merino lambs. A total of 48 lambs, over two consecutive years (n=24 per year), was randomly allocated into Spirulina supplementation level (CONTROL, LOW, MEDIUM, HIGH) groups and balanced by sire breed (Black Suffolk, Dorset, Merino, White Suffolk) and sex (ewes, wethers). Lambs had ad libitum access to a basal diet of pasture and Lucerne hay, and had Spirulina supplement daily throughout the 42-day feeding trial, after a 21-day adjustment phase. Mid-side wool samples were removed from each lamb at the beginning and end of the feeding trial and commercially analysed for quality. The wool quality data were transformed into ‘wool trait change over the feeding trial period’ by the difference between the wool measurements at the beginning and end of the experiment prior to statistical analysis. Wool clean fleece weight (CFW) was significantly influenced by level of supplementation, basal diet and sex (P<0.05). Spirulina-supplemented lambs on Lucerne hay basal diet elicited higher CFW responses which increased with supplementation level. CONTROL lambs had the lightest CFW regardless of basal diet. Sire breed and sex variations were significant (P<0.05) as wool from Suffolk-sired lambs and ewes had the highest fibre diameter coefficient of variation (CV) and CFW, respectively. CV and fibre diameter standard deviation were the most affected wool traits by significant interactions between Spirulina supplementation level, sire breed and sex. It was concluded that Spirulina supplementation with Lucerne hay basal diet improves wool CFW without compromising wool fibre diameter, comfort factor and spinning fineness in dual-purpose prime lambs.