Ultrasensitive leaky-lens-antenna coupled microwave kinetic inductance detectors (KIDs) for 1.5 THz space-borne polarimeters are demonstrated. These detectors are suitable for large scale, high performance imaging polarimetric instruments for applications such as measurement of interstellar dust in the far-infrared. Results of the demonstration show good optical beam quality with high Gaussicity of about 85% and the integrated cross-polarization ratio of about –21.5 dB for 1 f#λ spatial sampling. This novel detector avoids the need for a polarization modulator. Incorporating neutral density filters (NDFs) limited the power per detector to around 500 fW, enabling these measurements to be taken on detectors with noise equivalent power of 5–7×10−20 W/√Hz in a low background.