This research investigates the optimum spatial scale of regional tourism cooperation based on the tourism flows spillover in Mainland China. Using panel data from 341 prefecture-level cities in Mainland China, spatial econometric modeling is employed to estimate the spillover effects to determine the optimum spatial scale of regional tourism cooperation. The findings reveal that there are spatial correlations of tourism flow. In addition, tourism economic fundamental factor, surrounding market factor, tourism transportation facility factor, and tourist attraction factors have a positive and significant impact not only on local tourism flows but also on the surrounding areas. Finally, it is found that the optimum spatial scale of the regional tourism cooperation in Mainland China is [5, 15] cities, whereas for the eastern, western, southern, and northern regions, the optimum scale of the regional tourism cooperation is [5, 21], [5, 26], [5, 26], and [5, 11] cities, respectively.