Lanthanum Modified Bentonite (LMB; Phoslock®) is used to mitigate eutrophication by binding phosphate released from sediments. This study investigated the fate of lanthanum (La) from LMB in water, sediment, macrophytes, and chironomid larvae in Lake Rauwbraken (The Netherlands). Before the LMB application, water column filterable La (FLa) was 0.02 µg L−1, total La (TLa) was 0.22 µg L−1. In sediment the total La ranged 0.03–1.86 g m−2. The day after the application the maximum FLa concentration in the water column was 44 µg L−1, TLa was 528 µg L−1, exceeding the Dutch Maximum Permissible Concentrations (MPC) of 10.1 µg L−1 by three to fourfold. TLa declined below the MPC after 15 days, FLa after 75 days. After ten years, FLa was 0.4 µg L−1 and TLa was 0.7 µg L−1. Over the post-application years, FLa and TLa showed statistically significant downward trends. While the LMB settled homogeneously on sediment, after 3 years it redistributed to 0.2–5.4 g La m−2 within shallow zones, and 30.7 g m−2 to 40.0 g La m−2 in deeper zones. In the upper 20 cm of sediment, La concentrations were 7–6702 mg kg −1 dry weight (DW) compared to 0.5–7.0 mg kg−1 before application. Pre-application anaerobic sediment release of FLa was 0.006 mg m−2 day−1. Three months after the application it was 1.02 mg m−2 day−1. Three years later it was 0.063 mg m−2 day−1. Before application La in plants was 0.8–5.1 mg La kg−1 DW, post-application values were up to 2925 mg La kg−1 DW. In chironomid larvae, La increased from 1.7 µg g−1 DW before application to 1421 µg g−1 DW after one month, 3 years later it was 277 µg g−1 DW. Filtration experiments indicate FLa is not truly dissolved free La3+ cations.