An expandable air filter. The air filter including a filter media assembly, an outer frame assembly, and at least one support member. The filter media assembly defines opposing, first and second major faces and a perimeter. The outer frame assembly is arranged about the perimeter. The support member is connected to the outer frame assembly and extends across the first major face. Further, the support member defines a line of separation at which the support member can be separated into first and second segment to permit expansion of the air filter. The filter media can form a plurality of pleats, with at least one pleat being free of bonding to the outer frame assembly. The unbonded pleat experiences expansion as the air filter transitions to an expanded state. The air filter can be configured for expansion in one or both of a length and width direction.