Folded reflectarray antenna features high gain, low profile, and low sidelobe level, but also faces several challenges in scannability, manufacturing, assembling, and integration with planar circuits, especially at millimeter-wave and terahertz frequencies. To alleviate these issues, this paper presents a slot antenna based on substrate integrated waveguide (SIW) to replace conventional horn. This SIW slot antenna is then inherently integrated with reflectarray elements in the same single-layered printed circuit board (PCB) for the first time. Moreover, this folded reflectarray is able to scan its narrow beams in 2-D, by controlling the polarizing grids. The whole reflectarray is fabricated by the commercial low-cost PCB process and can be highly integrated with planar active circuits. Measured results show that the proposed reflectarray achieves a 2-D scanning zone of 14° $\times$ 14°, a gain of 27.4 dBi, and a cross-polarization level less than −24 dB. The proposed folded reflectarray is promising for several application scenarios including millimeter-wave high-resolution imaging and fifth-generation communications.