Time series measurements of the temperature and exit velocity of hydrothermal effluent suggest that seafloor hydrothermal systems are modulated by tidal processes. Here we apply the theory of poroelasticity to predict the magnitude and phase of tidally induced changes in the temperature and flow rate of hydrothermal effluent at the seafloor. We construct a model in which the steady state upwelling of buoyant fluid in the crust is modulated by tidal loading of a one-dimensional seafloor by the overlying water column. The nature of the solution is controlled by the relative magnitudes of three length scales. These are (1) the depth H of the heat source below the seafloor, (2) the skin depth D over which pore pressure signals can diffuse during one tidal cycle, and (3) the advective length scale A over which the upwelling flow advects thermal signals during one tidal cycle. We consider the likely magnitude of the parameters in a real system as well as the limitations of a one-dimensional representation of that system. We then discuss how observational data on the magnitude and phase lag of temperature and flow rate could be used to constrain the subseafloor parameters that govern hydrothermal circulation within the seafloor.