Mitochondria typically move non‐randomly due to attachment to cytoskeletal components. We investigated whether variation in ambient salinity affects mitochondrial movement in coelomocytes (nucleated “blood cells”) from the estuarine annelid Glycera dibranchiata using high‐resolution fluorescence video microscopy of live coelomocytes labeled with MitoTracker Green FM. Cells were exposed to normal seawater (1000 mOsm/L) or seawater diluted to 750, 500 and 250 mOsm/L (all with 10 mM HEPES and 0.1% glucose). Cell area and mitochondrial motion were analyzed in 100 cells and 400 mitochondria in a split plot, fully nested design (5 mitochondria x 5 cells x 4 worms x 4 dilutions). Surprisingly, mitochondrial movement at 1000 mOsm/L was continuous and random, characteristic of Brownian motion. Decreasing osmolarity proportionally increased the cell volume (P<0.001), as expected, but also increased the length of each mitochondrial track (P<0.0001) and the average total displacement (P<0.001), with mitochondrial movement at 250 mOsm/L being 2.8‐fold greater than at 1000 mOsm/L. Furthermore, the change in the mitochondrial diffusion coefficient was 3‐fold greater than that predicted by the direct effect of salinity on water viscosity. We propose that changes in the composition of intracellular organic osmolytes associated with salinity adaptation substantially affect the motion of these organelles.