We propose and experimentally demonstrate an in-line microfluidic sensor based on a dual S-taper multimode fiber interferometer (MFI) for glucose sensing. The dual S-taper MFI was fabricated by splicing two S-shaped fiber tapers with a commercially available fusion splicer. To realize in-line microfluidic sensing, the sensor was encapsulated into a capillary with the inlet and outlet to pump in and out the glucose sample solution using a syringe. Fourier frequency spectra of the transmission spectra under air and deionized water environments showed that multiple high-order modes simultaneously participated in the modal interference process. Experimental results indicated that the interference dip wavelength sensitivity reached 0.276  nm  /    (  g  /  dL  )   for the glucose concentration ranging from 0 to 25.0  g  /  dL. Our proposed glucose sensor has several advantages such as a compact structure, ease of fabrication, and low cost, which make it a promising candidate for in-line glucose sensing and other microfluidic sensing applications.