In this letter, a compact multi-functional antenna with full-polarization reconfigurability and continuous frequency tunability is proposed for integrated sensing and communication (ISAC) systems. To implement the multifunctionality, the concept of mode switch (MS) is introduced and theoretically investigated using a transmission-line model. Analytical results reveal that the working states of degenerate modes of a square patch, i.e., TM01 and TM10 can be independently controlled by MS. When MS is switched as capacitive loading, the mode is turned ON and its operating frequency can be tuned by varactors. When MS is switched as resistive loading, the mode is turned OFF and its RF energy is absorbed by resistors. On this basis, the desired multifunctional reconfigurability is simply realized on a square patch without extra couplers or phase shifters, making the design extremely compact in size and simple in realization. In final, the antenna is fabricated and measured. The results validate fullpolarization states, i.e., orthogonal linear polarization and dual sense of circular polarization, with a tunable operating frequency band of 1.65 GHz−2.15 GHz, demonstrating great potential for application in ISAC systems.