This paper tries to explore the concept of purification of the soul (tazkiyat al-nafs) and its relationship to happiness according to Ibn Qayyim al-Jauziyyah. Ibn Qayyim explained that the soul (nafs) is the essence of man with the ability to incline to good or evil. The soul was created with flaws and imperfect, but it will become perfect with the purification (tazkiyatun nafs). This research is a library study and the data collected using documentary techniques. The collected data were analyzed utilizing analytical-descriptive method. This study concludes the following. First, the purification of the soul according to Ibn Qayyim is not only to purify the human soul, but also to purify his faith. Second, Ibn Qayyim’s method of purifying the soul is by takhalli (emptying the soul) from all impurities of sin and tahalli (filling the soul) with obedience to Allah in the form of the main practices to achieve soul perfection. Third, happiness according to Ibn Qayyim is the happiness of the spiritual heart (nafs) as well as the happiness of useful knowledge. The highest and greatest happiness is the happiness of the heart in achieving ma’rifatullah. Fourth, the relationship of purification of the soul with happiness according to Ibn Qayyim is like a mirror in which there is a law of causality (cause and effect). The more the mirror of the heart is cleansed of the rust of sin, the image of His Throne will be clearer and more visible, so that it leads to ma’rifatullah.