There are a few different ways to answer this question, as the most popular soccer clubs in the world can be measured in a few different ways. One way to measure popularity is by looking at social media followings. As of June 2020, the three most popular soccer clubs on Facebook are Real Madrid (120 million likes), Barcelona (114 million likes), and Manchester United (73 million likes). On Instagram, the top three soccer clubs are again Real Madrid (71 million followers), Barcelona (67 million followers), and Manchester United (55 million followers). Interestingly, there is one club that cracks the top three on both platforms - Liverpool FC - who sit second on Facebook with 56 million likes and third on Instagram with 46 million followers.
Another way to measure the popularity of soccer clubs is through television viewership. In Europe, the English Premier League was watched by an average of 495 thousand viewers per match in 2019/20, while La Liga had an average of 398 thousand viewers per match over the same timeframe. In terms of individual clubs, Manchester United had the highest average viewership across all European leagues with 1.14 million people tuning in to watch their matches on TV. This is followed by Liverpool FC with 896 thousand viewers and Chelsea FC with 842 thousand viewers.
Finally, another metric for measuring popularity is merchandise sales. According to a report from October 2019, Nike sold more jerseys than any other brand worldwide during that year. The top selling Nike jersey belonged to Cristiano Ronaldo of Juventus, while Lionel Messi’s Barcelona jersey was second and Neymar’s Paris Saint-Germain shirt rounded out the top three best sellers worldwide. However, when looking at team sales rather than individual player jerseys, Real Madrid once again topped the list as they sold an estimated 1.6 million shirts during that year - 400 thousand more than their nearest rivals Barcelona who sold 1.2 million shirts over the same period.