Smartphones provide users much more than a mobile phone. For this reason, smartphones have become very important in our lives. In this context, the aim of this study was to investigate the effect of smartphone addiction on loneliness levels and academic achievement of Z generation high school students. In 2018-2019 academic year' spring semester, 490 high school students voluntarily participated to this study. As well as demographic information, Smartphone Addiction Scale and UCLA Loneliness Scale were used. In the analyses of the obtained data, IBM SPSS Statistics 24 was used. The relationship between variables was analyzed using Pearson's Correlation analysis. To determine the effect of smartphone addiction on high school students' loneliness and academic achievements, linear regression analysis was used. A positive relationship between Z generation high school students' smartphone addiction and right of monthly GB usage for smartphone, daily internet usage duration, and loneliness levels was determined, whereas there was a negative correlation between their academic achievements. Moreover, it was found that smartphone addiction of Z generation high school students predicted their loneliness and academic achievement at the level of ~4% and ~1%, respectively. These results show that high school students should be saved from smartphone addiction. Thus, loneliness levels can be reduced and academic achievement can be increased.