Eutrophication of enclosed coastal bays often results in anoxic bottoms and reduced sediment phosphorus (P) binding capacity. In the brackish bay Björnöfjärden, Baltic Sea, injection of aluminium (Al) solution into the euxinic sediment increased P binding capacity and reduced the flux of P to the water mass. Being the first Al treatment reported for a brackish water body, an extensive monitoring of P accumulation, together with measurements of the distribution of the added Al in the sediment profile, were performed over a decade. Peaks in Al-P formation were detected between 6 and 10 cm in the sediment profile and were accompanied by a corresponding increase in TP content, reaching 3.5 g P/m² after a decade. Al-P formation leveled out at 0.6 mg P/m² day over the first decade after treatment. Only a share of the added Al was associated to the increasing Al-P pool over time, resulting in an added Al to formed Al-P relation of about 7:1 (by weight). The rest of the added Al was distributed in the sediment profile, apparently not involved in P retention. We conclude that Al addition to Björnöfjärden sediment has effectively increased sediment P retention and accumulation over the decade following treatment and concerns, such as its function in salty or brackish environments and the stability of the Al-P bound in the sediment over time do not appear to be an issue.