This study was conducted to test interpersonal, attitudinal, and sexual predictors of sexual assertiveness in a Spanish sample of 1,619 men and 1,755 women aged 18-87 years. Participants completed measures of sexual assertiveness, solitary and dyadic sexual desire, sexual arousal, erectile function, sexual attitudes, and frequency of partner abuse. In men, higher sexual assertiveness was predicted by less non-physical abuse, more positive attitudes toward sexual fantasies and erotophilia, higher dyadic desire, and higher sexual arousal. In women, higher sexual assertiveness was predicted by less non-physical abuse, less solitary sexual desire and higher dyadic sexual desire, arousal, erotophilia, and positive attitudes towards sexual fantasies. Results were discussed in the light of prevention and educational programs that include training in sexual assertiveness skills.