A photonic-assisted microwave frequency divider that is able to perform frequency division with a tunable division factor is presented. It is realized based on the regenerative approach in which frequency mixing and filtering operations are implemented using a dual-parallel Mach-Zehnder modulator (DPMZM) and an optical filter in an optoelectronic oscillator loop. The frequency division factor can be tuned by controlling the optical filter passband, the round-trip gain, and the time delay of the optoelectronic oscillator loop. The proposed approach is analyzed theoretically and verified experimentally. The phase conditions to achieve frequency division for a given division factor are analyzed. Experimental results demonstrate, for the first time, a photonic-assisted regenerative microwave frequency divider with a tunable frequency division factor of 2 to 6. One key application of a microwave frequency divider is to improve the phase noise performance of a microwave signal. In our experiment, the phase noise is reduced by 16.2 dB when a microwave signal is frequency divided by 6 times.