The genetic systems ofPaspalumspecies have not been extensively studied. We analyzed the ploidy, reproductive mode, mating system, and fertility of fourPaspalumspecies—Paspalum durifolium,Paspalum ionanthum,Paspalum regnellii, andPaspalum urvillei. An analysis of 378 individuals from 20 populations of northeastern Argentina was conducted. All populations of the fourPaspalumspecies were pure tetraploid and had a sexual and stable reproductive mode. However, some populations ofP. durifoliumandP. ionanthumshowed low levels of apospory. Populations ofP. durifoliumandP. ionanthumhad low seed sets under self-pollination but were fertile under open pollination, showing that self-incompatibility likely caused self-sterility. In contrast, populations ofP. regnelliiorP. urvilleishowed no evidence of apospory, and seed sets in both self- and open pollination conditions were high, suggesting that they are self-compatible due to the absence of pollen–pistil molecular incompatibility mechanisms. The evolutionary origin of the fourPaspalumspecies could explain these differences. This study supplies valuable insights into the genetic systems ofPaspalumspecies, which could have implications for their conservation and management.