In this letter, a novel slot-loaded high-gain circular patch antenna is proposed to realize reconfigurable ±45° polarizations. Initially, a pair of y-oriented open slots 1 is loaded to the edges of a TM31-mode circular patch to manipulate its current distribution, which contributes to desired broadside radiation and high directivity. Second, two x-oriented slots 2 and four fan-shaped slots 3, which are mostly parallel to current vectors, are introduced to reconstruct the patch with the least perturbation. By controlling the on/off states of the PIN diodes planted at two ends of each of slots 3, reconfigurable orthogonal ±45° polarizations are achieved. Moreover, enhanced polarization discrimination is realized by optimizing the connection strategy between the patch's segments. The simulated and measured results demonstrate that the proposed antenna can achieve reconfigurable ±45° polarizations with 5.7% impedance bandwidth, 10.3 dBi realized gain, more than 25 dB polarization discrimination, and 23 dB backside radiation suppression.