ABSTRACT Materials based on polyethylene terephthalate (PET) and polyethylene terephthalate glycol (PETG) are in high demand for numerous practical applications. Their physico‐chemical properties depend heavily on the crystallinity degree (CD). Thus, it is important to be able to evaluate this parameter quickly, non‐destructively and with sufficient spatial resolution and accuracy. In this work, we propose to use Raman spectroscopy to assess the CD of PET and PETG. We examined six commercial samples of PET and PETG from different manufacturers, as well as two commercial dental sheets intended for the production of dental aligners. The CD of these samples, measured by differential thermal analysis (DTA), varied in the range of 0%–32%. It has been shown that the intensity ratios of the PET Raman bands I 858 /I 1615 , I 1096 /I 1615 , I 1729 /I 1615 , and I 2962 /I 3082 depend significantly and linearly on the CD. The method is applicable for granules, films, and manufactured articles with complex shapes.