Globally, recent studies report increases in Δ-9-tetrahydrocannabinol (THC) concentration in seized samples of cannabis for human consumption. This is important, because use of cannabis with a high concentration of THC has been linked to a number of adverse health outcomes. The objective of this study was to assess recent changes in the composition of seized cannabis resin in Denmark by (a) examining THC concentration in samples from Danish forensic laboratories from 2000 to 2017 (N = 430) and (b) examining cannabidiol (CBD) concentration and the THC:CBD concentration ratio in samples from the forensic laboratory in Western Denmark from 2008 to 2017 (N = 147). Cannabis resin samples were analyzed using a gas chromatographic analysis with flame ionization detection quantifying the total THC and CBD concentration. Results showed that the THC concentration increased 3-fold from 2000 (mean: 8.3%) to 2017 (mean: 25.3%). Significant increases occurred in all areas of Denmark. After 2011, we found a dramatic increase in cannabis resin samples with high THC concentration and the near disappearance of cannabis resin samples with medium- and low THC concentration. Furthermore, the THC:CBD concentration ratio increased significantly from 1.4 in 2008 to 4.4 in 2017. Whereas THC concentration increased, CBD concentration remained stable at 6%. In conclusion, the THC concentration of cannabis resin, and THC:CBD concentration ratio, have increased dramatically in Denmark, potentially leading to higher risk of harm to users. Policymakers, treatment professionals, and educators should be aware of this change.