OceananigansLagrangianFilter.jl filters simulation data generated by Oceananigans.jl simulations. The package provides methods for both offline (post-simulation) and online (in-simulation) filtering to remove motions that are high frequency in the Lagrangian frame. This is useful, for example, for removing waves from simulations when there is strong Doppler shifting, or when the wave displacement is large. OceananigansLagrangianFilter.jl solves sets of partial differential equations using Oceananigans solvers to find the filtered fields. It makes use of efficient sum-of-exponential filters (see Minz et al. 2025, Physical Review Fluids) to find the filtered variables at every time in the simulation, not just at one given reference time.