Celtic and Rangers have been among the top 20 most-watched clubs in the world for the past five years.
The CIES Football Observatory analyzed attendance in 51 national leagues in 42 countries and found that Celtic was in 16th place in the five years to 2018, Rangers two places lower.
The study also found that Celtic accounted for 36.5% of Scotland's total attendance during this period – more than any other club in the study.
Rangers had 27.4%, which was the eight highest percentage in the study. That was even though the Ibrox Club spent two of those five seasons in the second row.