Natural convection in a two-dimensional, square porous cavity filled with a nanofluid and with sinusoidal temperature distributions on both side walls and adiabatic conditions on the upper and lower walls is numerically investigated. The flow is assumed to be slow so that advective and Forchheimer quadratic terms are ignored in the momentum equation. The applied sinusoidal temperature is symmetric with respect to the midplane of the enclosure. Numerical calculations are produced for Rayleigh numbers in the range of 10- in comparison with other authors. The present models, in the form of an in-house computational fluid dynamics code, have been validated successfully against the reported results from the open literature. It is found that the results are in very good agreement. Results are presented in the form of streamlines, isotherm contours, and distributions of the average Nusselt number.