Assertiveness is the ability to express your thoughts, beliefs and opinions without the feeling of internal tension, that is why it is a desired trait of character. One of the examples of sports disciplines in which assertiveness can play a desired role is karate. One of the aims of this paper was to answer the question, whether the act of doing karate influences the level of assertiveness among people who practice it and to compare the results with the level of assertiveness declared by people who do other sports. The method applied in this paper was the diagnostics survey, while the tool was the questionnaire form based on the Rathus Assertiveness Scale. 50 students who practice karate on regular basis and 50 students who do other sports participated in the research. It is undeniable that the level of assertiveness among people who practice karate is higher in comparison to people who do other sports. Karate doers, both men and women obtained better results compared to the second group of sportsmen/women. This regularity can be observed among men, but it is more perceptible among women. As the research presents, people who attend karate trainings at least three times a week are characterized by a higher level of assertiveness in comparison to those who attend the training once a week.