An individual's assessment of the results achieved by analyzing how far his behavior in fulfilling his ideal is a picture of one's self-esteem. Low self-esteem is an individual's assessment of personal originating from an individual's assessment or from outside the individual obtained by analyzing behavior with an ideal self, in the form of negative feelings towards the ability of the individual itself, in overcoming the problem of patients with low self-esteem can be given cognitive therapy. This study aims to determine the effect of cognitive therapy on changes in self-esteem in patients with low self-esteem at Prof. Dr. M. Ildrem Mental Hospital in Medan. The design of this study is a Quasi-experimental group pre-test-post test design without control. The population in this study were all patients with low self-esteem as many as 47 people, with a sample of 22 respondents with a purposive sampling technique and statistical tests used the mc-Neymar test. The results of this study before being given cognitive therapy all respondents experienced low self-esteem of 22 respondents (100%) and after being given cognitive therapy, respondents' self-esteem became high as many as 19 respondents (86.4%) and compressed the value of p = 0,000 <0.05. The conclusion of this study is that there is a significant influence by providing cognitive therapy to self-esteem in patients with low self-esteem.