Biochar, an eco-friendly soil amendment, holds promise for remediating contaminated soils, yet its impacts on coastal wetland soils under combined microplastic (MP) and heavy metal (HM) pollution remain underexplored. This study examined the efficacy of 2% Spartina alterniflora-derived biochar (BC) in rehabilitating soils co-contaminated with cadmium (Cd) and two MPs—polyethylene (PE) and polylactic acid (PLA)—at 0.2% and 2% (w/w). The results indicated that biochar significantly elevated soil pH (8.35–8.43) and restored electrical conductivity (EC) to near-control levels, while enhancing organic matter content (up to 130% in PLA-contaminated soils), nutrient availability (e.g., phosphorus, potassium), and enzyme activity. Biochar reduced bioavailable Cd by 14–15% through adsorption and ion exchange. Although bacterial richness and diversity slightly declined, biochar reshaped microbial communities, enriching taxa linked to pollutant degradation (e.g., Proteobacteria, Bacteroidota) and upregulated functional genes associated with carbon, nitrogen, and sulfur cycling. Additionally, biochar boosted Suaeda salsa (S. salsa) biomass (e.g., 0.72 g/plant in A1B) and height (e.g., 14.07 cm in E1B) while reducing Cd accumulation (29.45% in shoots) and translocation. Remediation efficiency was most pronounced in soils with 0.2% PLA. These findings bridge critical knowledge gaps in biochar’s role in complexly polluted coastal wetlands and validate its potential for sustainable soil restoration.