Abstract Despite the fact that a small uncertainty in PDMS Poisson's ratio leads to significant errors in traction force microscopy, there is a clear lack of data for PDMS films at the scale of 100 μm, a relevant size scale frequently employed in cell mechanics studies. Equally important is the need for consideration of the viscoelastic nature of PDMS, as no mechanical property - including Poisson's ratio - can be taken as a time-independent constant. The foremost challenge for addressing these issues is the difficulty of carrying out stress relaxation tests on miniature PDMS samples accompanied by non-contact strain measurement with a very high spatiotemporal resolution. This study introduces such a stress relaxation platform incorporating i) the proper means for the application of necessary boundary conditions, ii) a high-precision in load measurement, and iii) a non-contact, local strain measurement technique based on single particle tracking. During stretching, images were recorded at a rate of 18 Hz with a 40 μm spatial resolution. Microsphere-embedded PDMS films as thin as 125 and 155 μm were prepared to study the Poisson's ratio by a local strain microscope. After tracing the displacement of microspheres by a single particle tracking method and using a strain mapping, Poisson's ratio for 155-μm-thick PDMS was found to decrease from 0.483 ± 0.034 to 0.473 ± 0.040 over a period of 20 min. For 125-μm-thick PDMS, this reduction took place from 0.482 ± 0.041 to 0.468 ± 0.038. Moreover, a non-monotonic reduction was observed in both cases. This negative correlation between Poisson's ratio and relaxation time was found to be statistically significant for both thicknesses with p