The environments animals inhabit vary greatly in structural complexity, both naturally and as a consequence of human disturbance. Structural complexity might affect communication by visual and other means, impair detection of potential partners, and affect sexual selection processes. Previous studies on shallow water fishes suggest that sexual selection can be relaxed when visibility is reduced. Here, we test whether habitat complexity affects mate search, mate choice, and the opportunity for sexual selection in the two-spotted goby, Gobiusculus flavescens, a marine fish with paternal care. In 2×2 m tanks, we established environments with low or high habitat complexity and introduced a mixed-sex group of fish (8 males, 8 females), which were allowed to breed. Two days later, we released additional (focal) ready-to-mate females in the tanks and observed female mate search and mating behaviors of both sexes. For females, habitat complexity negatively affected rate of movement, encounter rate with males, courtship rate, and time until mating. For males, habitat complexity resulted in fewer cases of multiple-male simultaneous courtships. Additionally, fewer courtship interactions were interrupted by male–male aggression in the complex habitat. However, these clear behavioral effects did not appear to affect the mating skew among males. Despite the absence of a difference in the opportunity for selection between treatments, we detected positive selection for male length in the open but not in the structurally complex environment. The results indicate that habitat complexity affects mating behaviors of both females and males and that a more structurally complex habitat might relax sexual selection.