Sailfish simulates accreting binary systems, including binary protostars, post-AGN stellar binaries, mass-transferring X-ray binaries, and double black hole systems. The binary components are "on the grid" rather than excised, and are evolved according to the Kepler two-body problem, modified to account for gravitational wave losses or self-consistent forcing from the orbiting gas. The solvers are shock-capturing and are second order accurate in space and time. Gravity is fully Newtonian. Thermodynamics can be treated using a gamma-law equation of state with a blackbody cooling term, or in the locally isothermal approximation, in which the gas temperature is set to a constant times the local free-fall speed. Sailfish is fully Cartesian and has extensive diagnostic capabilities to facilitate accurate calculations of gas-driven orbital evolution or the extraction of electromagnetic disk signatures. The code is extremely efficient, reaching more than one billion zone updates per second on an NVIDIA A100 GPU, enabling extremely high resolution of complex flows around the binary components.