A single-feed equilateral-triangular microstrip antenna has been designed with truncated corners for diverse radiation polarizations. The proposed antenna is excited by a microstrip line placed parallel to the oblique side of an equilateral-triangular patch. The antenna exhibits linear polarization (LP), left-hand circular polarization, or right-hand circular polarization (RHCP) depending on whether the three identically sized corners of the triangular patch are cut. A novel reconfigurable microstrip antenna with selectable polarization is proposed based on this first antenna. This second antenna has a simple structure consisting of a corner-truncated triangular patch, a small triangular conductor, an asymmetrical microstrip feed line, and a biased PIN diode between the patch and the triangular conductor. It produces LP or RHCP depending on the bias voltage. The measured results show that this antenna has a low cross-polarization level when operated in the linear state and a good axial ratio in the circular state.