Sexual dimorphism between females and males can sometimes be related to particular mating behaviors. In pseudoscorpions, very little is known about the relationship between sexual size dimorphism (SSD) and reproductive behavior. This paper describes sexual dimorphism and the mating behavior patterns of Lustrochernes argentinus (Thorell, 1877), a Neotropical species of Cheliferoidea. We measured different body structures and appendages of 14 individuals of each sex, with the values compared with a T-test. We also analyzed the sexual behavior repertoire occurring during mating between nine couples. We found SSD for several structures: the fixed and movable finger width, and the angle between IV coxae are larger in females, in contrast with the gap between fingers which is bigger in males. The analysis of sexual behavior patterns in this species showed that the reproductive behavioral repertoire is structured in three phases: Phase I (examination), Phase II (spermatophore deposition) and Phase III (sperm transfer) with a mean duration of 2.2 min, 5 min, and 7.5 min, respectively. Some structures that showed SSD, and also the composition and duration of different behavioral phases, are different from those of other species of the family Chernetidae. We discuss the results based on current knowledge in pseudoscorpion biology with prospects of further studies studying sexual selection.