Flavonoids are ubiquitous compounds commonly found in vegetables, fruits and other plant foods. Although not considered nutrients per se, consumption of various flavonoids is associated with established health benefits. Their biosynthesis, and therefore concentrations, are influenced by genetic, geographic and environmental conditions. Flavonoid content in foods can be seasonal, potentially influencing their total intake and biovailability. In view of the potential role of flavonoids in human health, studies published over an 11-year period (2009 to 2020) investigating links between flavonoid content and season in edible and medicinal plants, were examined. The limited studies to date focus on a small range of plant species. Within this, there is consistent evidence that flavonoid content varies according to season, particularly in relation to plant genotype and environmental conditions such as temperature, geographic location, light conditions/UV radiation and drought/water stress. Seven studies detected highest total flavonoid content at the end of winter and lowest in midautumn. From the included studies, rutin was the most commonly studied flavonoid, showing its highest levels in both spring and winter. These findings suggest studies on flavonoid intake should include seasonal considerations. Further studies on seasonal variations of common dietary flavonoids are warranted to enable such studies.