Excessive internet gaming can lead to internet gaming disorder (IGD). Internet games often include the ability to make in-game purchases. For example, loot boxes are random video game items that can be purchased with real-world money. This study examined whether unplanned in-game purchases are linked to problematic online gaming and mental health issues. A total of 1,052 Japanese high school students were surveyed. They were asked questions about their online gaming, in-game purchases, gaming problems and mental health. Adolescents who made unplanned purchases had a greater degree of problematic online gaming than those who planned their purchases. Also, participants who made unplanned purchases of loot boxes had more symptoms of depression and gaming problems related to preoccupation and overuse than those who made other unplanned purchases.