We propose and demonstrate an ultrasensitive integrated photonic current sensor that incorporates a silicon-based single-mode-multimode-single-mode waveguide (SMSW) structure. This kind of SMSW structure is placed over a direct current carrying power resistor, which produces Joule’s heat to change the temperature of the SMSW and further results in the change of the effective refractive index between different propagating modes. Interference occurs when the modes recombine at the second single mode waveguide. Finally, the current variation is measured by monitoring the shift in the output spectrum of the multimode interferometer. In low current, the wavelength shift has almost linear dependence: $\mathrm{\Delta }\lambda \propto I_c$. This effect can be used as a current sensor with a slope efficiency of 4.24 nm/A in the range of 0–200 mA.