Sensing technologies based on terahertz waves have immense potential due to their non-destructive, transparent, and fingerprint spectral response of several materials that are opaque to other parts of the electromagnetic spectrum. Terahertz metasensors reported so far merely exploit the fringing electric field on the top of the subwavelength resonators. Here, we experimentally demonstrate an ultrathin flexible terahertz metamaterial sensor on a low refractive index substrate which enables sensing of analytes from the top and bottom surfaces of the metamaterial, opening up avenues for dual-surface sensing of analytes with fringing resonant fields on both front and rear sides of a metasurface. Since most of the real-world objects have 3D curvatures, the reported flexible metasensor with large mechanical strength and stability in free space would be an ideal platform for ultrasensitive sensing of dielectrics, chemicals, and biomolecules of extremely low concentrations with dual non-planar surfaces. © 2017 Author(s).