Renewable energy sectors have been rapidly growing over the last three decades due to the environmental concerns regarding fossil fuels and increasing demand of energy by human. Among those, offshore wind farms are one of the most attractive and promising technologies for clean energy production due to the strong and steady offshore winds, less turbine fatigue, less visual and space limitations compared to onshore wind farms. Rapid development of offshore wind farms, which is expected to reach 70% by 2030, can effect on marine ecosystems and organisms. Hitherto, different studies have comprehensively discussed the potential impacts of offshore wind farms on marine habitats; however, they are just potential and rarely validated through observations. This review focuses on the proved environmental impacts of offshore wind farms gained from post-construction environmental monitoring programs. Particularly, this study provides significant insights on: 1) the area and time span over which biological effects may occur, 2) responses to disturbance by different target organisms; 3) quantification of short/long-term effects; 4) recovery from impacts in the long term. The monitoring studies showed little or only local impacts of offshore wind farms on the marine environment, either during their construction or the operational phases. However, further research is needed to answer whether synergies of little and local impacts may determine consequences at the population level. As the number and size of offshore wind farms increase it is necessary to consider consequences at the population level as well as cumulative impacts of these activities on marine ecosystems.