In the present study we investigated the reliability and validity of an Implicit Association Test of sexual assertiveness (the SA-IAT) in a sample of young adults (n = 159). The D600 algorithm was used to calculate implicit sexual assertiveness scores. Explicit sexual assertiveness was measured using a selection of items from the Hurlbert Index of Sexual Assertiveness. Personality traits were assessed using the revised, short version of the Eysenck Personality Questionnaire. The internal consistency of the SA-IAT was evaluated based on split-half reliability, and found acceptable with α = 0.61 for the practice trials, and α = 0.70 for the test trials, after correction for attenuation. Convergent and divergent validity were evaluated using correlation analysis. Correlation with explicit sexual assertiveness was found to be low, as expected. Divergent validity of the SA-IAT was evaluated against the personality traits of extraversion, neuroticism, and social desirability. Except for a significant correlation of implicit sexual assertiveness with extraversion in the full sample and the female subsample, implicit sexual assertiveness and personality traits were not found to share variance, as expected.