Using spent coffee grounds (SCG) as soil amendment could serve as a solution to two major issues by revalorising an abundant residue while reducing agriculture’s dependence on harmful chemical fertilisers to feed a rapidly growing population. However, in light of the challenges SCG poses to soil quality and plant growth due to their acidity, phytotoxicity, and often high C:N ratio, this research aimed to assess how composting SCG before applying them could impact their efficiency for soil amendment. Caffeine content, pH, and soil respiration were measured, and a germination experiment was conducted with samples of SCG that had been composting for 0 to 4 weeks as well as with mixtures of composted SCG and soil at a rate of 1:10. The results found that composting SCG for only 4 weeks significantly increased pH and decreased the concentration of phytotoxic substances. Adding SCG to the soil at a rate of 1:10 increased soil respiration and had no significant impact on the germination of Lepidium sativum seeds, but composting also had no significant impact on the results when applied at this ratio. Overall, SCG has high potential to be applied for soil amendment and composting is a simple and efficient way to reduce their acidity, phytotoxicity, and prevent nitrogen immobilisation.