The sludge produced by rice parboiling wastewater treatment plants has substantial amounts of nutrients and organic matter, therefore requiring proper environmental disposal. One option is to use it as an alternative substrate for the production of forest species seedlings. This study aimed to assess the viability of using the sludge from rice parboiling wastewater as an alternative substrate for the production of Araucaria angustifolia (Bertol.) O. Kuntze seedlings. A. angustifolia seeds were kept in vermiculite until sprouting, then transferred to polyethylene bags filled with five different substrates (treatments): 100% sand (T1); 75% sand and 25% sludge (T2); 50% sand and 50% sludge (T3); 25% sand and 75% sludge (T4); 100% sludge (T5). Each treatment had 10 replicates (bag with one seedling). Shoot system growth, hypocotyl diameter, root system growth and biomass were evaluated at 94, 180, and 300 days of growth. Data were subjected to Analysis of Variance (ANOVA) followed by Tukey test with a significance level of 5%. Seedlings’ growth was similar on treatments containing 25%, 50%, and 75% sludge, while 100% sand and 100% sludge resulted in less growth at 300 days. In conclusion, the sludge from rice parboiling wastewater is suitable as an alternative substrate, and the ratio of 25% sand + 75% sludge could be recommended, for exploit larger amounts of the residue.