Rhizophora mangroves play an important role in maintaining shoreline stability and supporting the sustainability of coastal ecosystems. The success of mangrove rehabilitation is strongly influenced by the quality of nursery media; however, commonly used media are still dominated by muddy substrates with low porosity and aeration, which are less optimal for root development and seedling growth. Nickel slag, an industrial by-product, has a coarse and porous structure and contains inorganic minerals such as CaO, MgO, and SiO₂, making it potentially suitable as a soil ameliorant. This study aimed to analyze the effects of nickel slag size and concentration on nursery media conditions and the growth of Rhizophora mangrove seedlings. The study was conducted for three months in North Maluku using a Factorial Completely Randomized Design with three nickel slag size variations, namely coarse (SNK), fine (SNH), and mixed (SNC), and four concentration levels of 25%, 50%, 75%, and 100% of the planting media, with five replications for each treatment. The observed parameters included survival rate, seedling height, number of leaves, chlorophyll content, number of root fibers, media pH, and the contents of N, P, K, Ca, Mg, and SiO₂. Data were analyzed using ANOVA followed by Tukey’s post hoc test at a 95% confidence level, while media chemical parameters were analyzed descriptively. The results showed that all treatments maintained a 100% seedling survival rate. The SNC 50 treatment produced the best response, with the highest seedling height (18.6 cm), number of leaves (3.2 leaves), chlorophyll content (64.36 SPAD units), and number of root fibers (19.8). The addition of nickel slag at moderate concentrations improved media conditions by enhancing porosity, aeration, and nutrient uptake, thereby better supporting the growth of Rhizophora mangrove seedlings.