With seven games to go in the season, Inter sit fifth, but a three-game winless run has allowed sixth-placed Parma to close the gap to just two points.
The dip in form has led to reports suggesting that Thohir will remove the 52-year-old from his position, but he insists he has full faith in Mazzarri.
"If I didn't have confidence in him, I would have let him go a couple of months ago," Thohir told Inter's official website.
"The players need to learn from the mistakes they made against Livorno (a 2-2 draw last weekend), we need to get our focus back on the game.
"At the end of the season we will review the players and the coach together to build Inter's future and that means preparing for next season."
Inter host relegation-threatened Bologna on Saturday looking for their eighth home win of the season in Serie A.
