Changes in the environment due to human activities are becoming increasingly common. A serious problem in aquatic environments is increased water turbidity due to phytoplankton algal growth. This may affect the breeding system of fishes, especially those with a visually based mating system. Here we show that increased turbidity affects sexual selection in the threespine stickleback (Gasterosteus aculeatus) through impaired possibility for visually based mate choice. In a laboratory mate preference and mate choice experiment on sticklebacks from the Baltic Sea, which is an area suffering from increased turbidity due to human activities, we found that females spent more time with and visited more often males in clear water than males in turbid water. For males in turbid water to receive the same amount of interest from females as males in clear water, they needed to court significantly more. Thus, turbid water induced selection for higher courtship activity. However, the final spawning decision of the females did not depend on water turbidity, which suggests that nonvisual cues determined the final spawning decision. Because visual cues are important in mate attraction, increased turbidity affects an important evolutionary force, sexual selection, which may have further consequences for the evolution of the sexual displays and preferences. Differences in visual conditions could hence be one factor that has lead to differences among stickleback population in the use of sexual signals.