A novel design approach to achieve harmonic suppression of a low-profile microstrip patch antenna (MPA) under tri-polarization is proposed. Herein, the tri-polarization consists of the left-handed circular polarization (LHCP), right-handed circular polarization (RHCP), and linear-polarization (LP). First, the resonant frequencies of the traditional MPA are deeply studied. It is demonstrated that a set of radiative modes are excited in a single patch resonator. In order to suppress a few undesired modes, an array of shorting vias is loaded around two edges of the quarter patch1. Then, a linear slot is introduced to move dual orthogonal modes closely to each other, resulting in achieving LHCP patterns with harmonic suppression. Similarly, both of RHCP patterns and harmonic suppression are acquired for the quarter patch2 via this method. After that, the shorting vias and slots are adopted on the half patch3, aiming to suppress or push up higher modes for LP generation and harmonic suppression. Lastly, all these three patches are integrated with each other for size reduction. Finally, the proposed MPA is fabricated and measured. The results illustrate that the antenna has satisfactorily gained LHCP, RHCP, and LP radiation patterns of around 2.65 GHz. Besides, the volume of the radiator (0.0058 λ03, λ0 is the free-space wavelength) is only half of the traditional counterpart (0.01λ03). Additionally, the port isolation is kept as large as about 22 dB in the operating band. Most importantly, the harmonic suppression of around 2.7f0 (f0 is the operating frequency) is successfully acquired without extra feeding circuits.