The hypothesis that supplementation with Spirulina will increase liveweight, growth and body conformation with significant interactions between sire breed and sex was tested using purebred Merino weaners and first-cross weaners from Merino dams sired by Dorset, Black Suffolk and White Suffolk rams under the same pasture-based management system. Our experimental objective was to evaluate the effects of varying levels of Spirulina supplementation, sire breed and gender on liveweight and body conformation traits. We utilized a complete randomized block experimental design balanced by 4 sire breeds, 3 supplementation levels and 2 sexes in which weaned prime lambs with an average liveweight of 37.6 ± 5.2 kg and body condition score of 3.1 ± 0.4 at 6 months of age were balanced by sire breed and gender and randomly allocated into 3 treatments (8 lambs per treatment) – the control group grazing without Spirulina (0%), low (10%wt/vol) and high (20%wt/vol) Spirulina. Lambs in the low and high Spirulina treatment groups were drenched daily with Spirulina prior to being released for grazing with the control group of lambs over a 6-week trial period, following a 3-week adjustment phase. Weekly measurements of chest girth, withers height, body length, body condition score and liveweight were taken. Mixed linear model procedures in SAS with sire breed, sex, Spirulina level and their second order interactions as fixed effects and sire as a random variable, were used for statistical analysis. Spirulina level significantly influenced lamb liveweight (P<0.018), body condition score (P<0.001) and body length (P<0.015). Lambs on Spirulina levels of 10% recorded the highest mean liveweight of 41.9 ± 0.7 kg. Spirulina levels of 20% did not significantly improve liveweight compared to the control group (0%). Highly significant sire breed interactions with Spirulina level (P<0.001) resulted in the heaviest (47.08 kg) and lightest (35.14 kg) average liveweights in Black Suffolk-sired crossbreds and purebred Merino lambs respectively, supplemented at the 20% Spirulina level. Body conformation (P<0.001) and liveweight (P<0.014) responses to Spirulina supplementation significantly varied between ewe and wether lambs. It was evident that a cost-effective supplementation strategy with Spirulina for optimal liveweight gains in weaner lambs was achieved at the 10% level. These findings will aid sheep farmers in making informed choices about appropriate sire breed and gender combinations in their enterprises when supplementing with Spirulina for growth improvement as a strategic pathway for the early attainment of market weights in prime lambs. We concluded that based on the empirical experimental evidence within the scope of this study, the tested hypothesis is acceptable.