Indonesia is the country with the largest nickel reserves and production levels in the world. Each ton of nickel production can produce eight tons of by-products in the form of ferronickel slag, which continues to increase due to the minimal recycling process of these by-products. This study aimed to determine the impact of changes in acid concentration and solid/liquid ratio on the leaching extraction of magnesium from ferronickel slag and characterize the transformation of ferronickel slag at each stage of the process. The research was conducted using the alkali fusion method and continued with water leaching and acid leaching using Hydrochloric Acid (HCl) as the solvent. The first step in the investigation was milling to get a powder with the particle size ≤127 µm. The sample powder was subsequently mixed with the Na2CO3 additive in a 50:50 (w/w) ratio and roasted for 60 minutes at 1000 °C. The water leaching procedure was then conducted for 60 minutes at 100 °C, a 1:10 (w/v) ratio, and a stirring speed of 400 rpm. The filtrate and residue were then separated using a filtration process. An acid leaching utilizing HCl with concentration variations of 0.5, 1, and 1.5 M, and solid/liquid ratio (s/l) variations of 1:20, 1:30, and 1:40 (w/v) was performed on the residue. The results of acid leaching were then filtrated again. The filtrate was then characterized by ICP-OES testing. Based on the analysis results, it can be stated that the percentage of magnesium extraction increases as the solid/liquid ratio (s/l) increases but decreases with the increase in HCl concentration. The optimum percentage of magnesium extraction is 64.12 %, which was achieved with the leaching conditions of a solid/liquid ratio (s/l) of 1:40 (w/v) and a HCl concentration of 0.5 M