Faecal microbiota transplantation (FMT) has been explored as a potential treatment for obesity, but its long-term effects on metabolic health remain unclear. Here, we report 4-year follow-up findings from a double-blind, randomised, placebo-controlled trial assessing FMT in adolescents with obesity (ACTRN12615001351505, Australian New Zealand Clinical Trials Registry). This unblinded follow-up study evaluated 63% (55/87) of the original participants (27 FMT, 28 placebo). There was no difference in BMI between the two groups, after adjusting for sex, age, diet, and physical activity (-3.6 kg/m, p = 0.095). However, FMT recipients showed clinical improvements in body composition and metabolic health compared to the placebo group. Specifically, FMT recipients had smaller waist circumference (-10.0 cm, p = 0.026), total body fat (-4.8%, p = 0.024), metabolic syndrome severity score (-0.58, p = 0.003), and systemic inflammation (-68% hs-CRP, p = 0.002) and higher levels of HDL cholesterol (0.16 mmol/L, p = 0.037). No group differences were observed in glucose markers, or other lipid parameters. Shotgun metagenomic sequencing revealed sustained long-term alterations in gut microbiome richness, composition and functional capacity, with persistence of donor-derived bacterial and bacteriophage strains. These findings highlight the potential relevance of FMT as a microbiome-augmenting intervention for obesity management and metabolic health, warranting further investigation.