The lockdowns resulting from the first wave of the COVID-19 pandemic impacted deeply on all life activities, including diet. We performed a systematic review to investigate changes in food intake, eating behaviours and diet quality during lockdown as compared to before. A literature search was performed using three electronic databases from inception until June 13, 2021. Observational studies evaluating changes in general populations during the COVID-19 pandemic lockdown were eligible. Out of 1,963 studies achieved from the search strategy, 95 met inclusion criteria (85 on adults, 10 on children/adolescents), and the majority were of high quality (72.6%). Most of the studies were web-based surveys using convenience sampling, mainly focused on variations in the consumption of foods and eating behaviours during lockdown, whereas only 15 studies analysed diet quality through dietary indices. On the basis of the definition of a healthful diet as reflected by a traditional Mediterranean diet, an increase in recommended foods such as fruit and vegetables, legumes, cereals and olive oil was observed, although a sharp decrease in fish intake and an increase in dairy products were documented. Accordingly, a reduction in foods that should be eaten less frequently was reported, namely, red and processed meat. However, a higher consumption of unhealthy foods (e.g., snacks and sweets) was also observed. Results indicated improved diet quality in Europe, especially among Mediterranean countries, with the exception of France, while a switching to poor nutrient patterns was observed in Colombia and Saudi Arabia. Analyses of eating behaviours suggest an increase in food intake, number of daily meals and snacking. In conclusion, changes in intake of major food groups, apart from fish intake, were in line with the definition of a traditional Mediterranean diet, indicating a consistent moderate improvement of dietary habits worldwide. This review protocol was registered at https://www.crd.york.ac.uk/prospero/ as CRD42020225292.