Alzheimer’s disease (AD) is a neurodegenerative disease characterized by deficits in episodic memory processing and spatial navigation. Especially, the egocentric neural representation of space, which is important for constructing an egocentric cognitive map, is essential for spatial navigation. However, how spatial navigation deficits relate to egocentric neural representation of space remains unclear. The retrosplenial cortex (RSC) is one of the regions affected by amyloidosis in the early stages of AD and is well known for its role in egocentric spatial representation. Specifically, it contains egocentric boundary cells and egocentric vertex cells. Thus, in this study, we investigated how RSC neurons are affected in AD by employing in vivo calcium imaging in GCaMP-expressing excitatory neurons in the RSC of both control mice and the 5XFAD mouse model of AD, while mice freely explored a square open chamber. To quantify the changes in egocentric spatial representation, we analyzed the preferred egocentric direction and distance of individual RSC neurons relative to environmental boundaries and vertices, from which the strength of egocentric vector coding (vector length) was computed. Our analysis revealed that the population of neurons that perform egocentric vector coding of environmental boundaries and vertices was significantly reduced in 5XFAD mice compared to control mice. Further analysis revealed that this disruption in egocentric vector coding of environmental geometry was specifically due to a deficit in egocentric distance coding, while egocentric direction coding remained unaffected in 5XFAD mice. Especially, while healthy control mice could perform vector coding of environmental geometry both proximally and distally, 5XFAD mice could only code for egocentric vectors at proximal distance to the environmental geometry. Together, our findings suggest that the spatial navigation deficits observed in AD may be related to a disruption in the ability to construct egocentric cognitive map that are independent of proximity to the environmental features.