In this paper, the long-term forecasting of the highest summer floods by means of commensurability method on the Danube River at Bratislava was carried out. The Bratislava city is the capital of Slovak Republic, as well as its major administrative and industrial center. In the past, the Bratislava suffered from dangerous floods. The highest floods occurred most frequently in the summer. Consequently, long-term forecasting of summer floods on the Danube River at Bratislava has an important scientific and practical significance. We used of dates of the highest summer floods for the period 1876-2018, as well as the information about historical highest summer floods, which occurred before the beginning of regular hydrometric observations. The commensurability method allows predicting of dangerous natural phenomena, including floods. It is characterized by the simplicity of the calculations and using of minimum input information array. We used four ways of forecasting: by the calculated value of commensurability; by the two-dimensional and three-dimensional graph of commensurability; by the time intervals between floods that have occurred in the past; and by the number of commensurability equations with three components. The presented results illustrate that the following highest summer floods can occur on the Danube at Bratislava in 2020, 2025 and 2030.