The aim of this paper work was to compare the returns and risks of the tourism sector of Mediterranean countries using Markowitz's model. Four countries were used for the analysis, Croatia, Italy, Spain and France. The purpose of this paper was to determine the difference between the tourism sector between similar countries, for which tourism is a popular activity, and to determine the impact of the COVID-19 crisis on the tourism sector of the observed countries. After the introduction, a review of Markowitz's model of portfolio optimization is presented in a theoretical part, furthermore, there is explanation of the importance of the tourism sector for a particular country. In the practical part of the paper, was given an analysis of shares according to modern portfolio theory in order to reach a conclusion of acceptance or rejection of hypotheses. After completed analysis, we made conclusion about a significant drop in yield during the COVID-19 pandemic compared to before. According to the graphical presentation of effective limits in 2019, for the same level of risk, the Italian tourism sector achieves the highest rates of return, followed by the French tourism sector, then the Spanish and finally the Croatian tourism sector with the lowest rates of return. While in 2020 the highest rate of return is achieved by the Italian tourism sector, but with a high level of risk, and the French tourism sector achieves lower returns with lower levels of risk, and dominates the Spanish and Croatian tourism sectors. The risk measured by standard deviation and VaR shows that shares of the Spanish tourism sector are the most risky, although they do not have the highest returns. The conducted analysis indicates the obvious impact of the COVID-19 pandemic on the tourism sector, and that the investment depends on the risk appetite of investors.