The design of a tri-band dual-polarization (TBDP) shared-aperture microstrip array antenna for synthetic aperture radar (SAR) applications is presented. It operates at L-, Sand X-bands with a frequency ratio of 1:2.8:8. This TBDP shared-aperture array is assembled by two L/S and L/X DBDP shared-aperture sub-arrays with one L-band dual-polarized sub-array. A prototype array has been fabricated and measured, which exhibits wide impedance bandwidth (VSWR ≤ 2) of 13.4%, 14.8%, and 16.8% in L-, S-, and X-bands, respectively. The measured array isolation is better than -27 dB in all three bands. The measured radiation patterns agree well with the simulation, confirming the cross-polarization level of below dB within the main lobe region, and the array scanning capability of ±27° in S- and X-bands. This array design approach can be further extended to shared-aperture arrays with more than three bands.