We demonstrate a novel method for measuring terahertz (THz) photoconductivity of semiconductors on length scales smaller than the diffraction limit at THz frequencies. This method is based on a near-field microscope that measures the transmission of a THz pulse through the semiconductor following photoexcitation by an ultrafast laser pulse. Combining back-excitation of the sample using a Dove prism, and a dual lock-in detection scheme, our microscope design offers a flexible platform for near-field time-resolved THz time-domain spectroscopy, using fluences available to typical laser oscillators. Experimental results on a thin film of gallium arsenide grown by metal organic chemical vapor deposition are presented as a proof-of-concept, demonstrating the ability to map the complex conductivity as well as sub-ps dynamics of photoexcited carriers with a resolution of λ/10 at 0.5 THz.