Among the phytocomplex components of Cannabis sativa L., cannabidiol (CBD) has a recognised therapeutic effect on chronic pain. Little is known about the veterinary use of CBD in dogs. Even less is known on the effects of CBD on dog behaviour, especially in shelters. The purpose of this study was to determine if CBD affects stress related behaviour in shelter dogs. The sample consisted of 24 dogs divided into two groups that were created by assigning the dogs alternately: 12 dogs were assigned to the treatment group and 12 to the control group. Extra virgin olive oil, titrated to 5% in CBD was given to treated group; the placebo consisted of olive oil only, dispensed daily for 45 days. Behavioural data were collected using the ‘focal animal’ sampling method with ‘all occurrences’ and ‘1/0’ methods for 3 h: before (T0), after 15 days (T1), after 45 days of treatment (T2) and after 15 days from the end of the treatment (T3). Treated dogs showed reduced aggressive behaviour toward humans following the treatment (Friedman Test: χ2 = 13.300; df = 3; N = 12; p =.004; adj. sig. p = 0.027), but the difference in the decrease of aggressive behaviour between the two groups was not significant (Mann–Whitney U test, T2–T0: Z = − 1.81; N = 24; p = 0.078). Other behaviours indicative of stress, such as displacing activities and stereotypes, did not decrease. Despite some non-significant results, our findings suggest that it is worth doing more research to further investigate the effect of CBD on dog behaviour; this would be certainly valuable because the potential for improving the welfare of dogs in shelters is priceless.