Large amounts of tanned leather waste can be utilized as a valuable raw material in the fertilizer industry by embracing circular economy principles. Due to their high collagen content, which provides an essential nitrogen source (14%), these wastes hold significant potential. However, to incorporate them into fertilizer formulations, leather waste must first be hydrolysed to produce gelatine. This study focuses on developing an optimal method for hydrolysing tanned leather waste. The experiments investigated the effects of several parameters - including the type and concentration of hydrolysis agent, temperature, and contact time - on the degree of hydrolysis achieved. Leather waste samples were treated with various hydrolysis agents (water, sulfuric acid, oxalic acid, potassium hydroxide, and ash) at different concentrations and temperatures ranging from 40°C to 110°C for periods between 60 and 160 minutes. Results indicated that basic hydrolysis was the most effective approach. Treating leather waste with 0.5 M KOH at 100°C for 120 minutes, with a stirring speed of 400 rpm and a 1:10 solid-to-liquid ratio, resulted in a degree of hydrolysis of 98.58%. Comparable results (97.25% hydrolysis) were obtained with a 1:2 ash-to-water leachate under similar conditions (100°C for 100 minutes, 400 rpm, 1:10 ratio). These findings demonstrate that both potassium hydroxide and ash leachate are highly effective agents for the hydrolysis of tanned leather waste, supporting their integration into sustainable fertilizer production.