A heat exchanger for an aircraft includes an inlet plenum housing defining an inlet plenum configured to receive a fluid and an outlet plenum housing defining an outlet plenum configured to discharge the fluid. Furthermore, the heat exchanger includes a core configured to heat or cool a first portion of the fluid, with the core defining a plurality of fluid passages extending from the inlet plenum to the outlet plenum. Moreover, the heat exchanger includes a passive bypass defining a passive bypass flow path fluidly coupled to and extending between the inlet plenum and the outlet plenum. As such, the passive bypass flow path is in parallel with at least a portion of the core such that a second portion of the fluid bypasses at least a portion of the core and flows continuously and unobstructed through the passive bypass flow path to the outlet plenum.