Recent developments in two-dimensional (2D) Ruddlesden-Popper perovskites have ushered in a new phase in optoelectronics through their promising physical, chemical, and optical properties. Thin films of 2D perovskites used in most optoelectronic applications contain many grains oriented either horizontally or vertically with respect to the substrate on which they grow; the orientation of such grains plays important roles, not yet fully understood, in their functionalities. Techniques to map the grain orientation in situ in 2D perovskite thin films are hence highly desirable. Here we report mapping of the grain orientation of thin films of the horizontal 2D perovskite, BA2PbI4, in which BA is butylammonium, with low-frequency polarized Raman microspectroscopy. We established a relation between the intensities of low-frequency (10-60 cm-1) polarized Raman lines of BA2PbI4 originating primarily from the PbI6 octahedra and varied orientations of a plate-like BA2PbI4 single crystal. This relation served to determine unambiguously the orientation of grains of micrometer size in a spin-coated BA2PbI4 thin film. Although this study is a case study of BA2PbI4, it opens a new window to obtain crystallographic information about 2D perovskite thin films in general and hence to understand their properties and functions.