Chitosan, a natural polysaccharide, is widely regarded as a biocompatible and potential carrier for controlled-release drug formulations. However, the potential effect of chitosan on the environmental behavior of contaminants is poorly understood, especially on chiral chemicals. In this study, the changes in bioavailability of the chiral herbicide dichlorprop to Chlorella pyrenoidosa by chitosan were investigated. The dissipation of (S)-enantiomer in Chlorella pyrenoidosa culture media without chitosan was faster than that of the herbicidally active (R)-enantiomer, whereas it was inversed to (R)-enantiomer being faster than (S)-enantiomer when chitosan was added into the media. In the absence of chitosan, the toxicity of (R)-enantiomer to Chlorella pyrenoidosa was more potent than that of the (S)-enantiomer. On the contrary, in the presence of chitosan, (R)-enantiomer was less toxic than (S)-enantiomer. These observations clearly suggest that chitosan changed the enantioselective bioavailability of dichlorprop. Fluorescence spectroscopic analysis showed that the interaction between chitosan and dichlorprop enantiomers depended greatly on the steric structure of dichlorprop, which offers a possible explanation as to why the addition of chitosan changed the enantioselective dissipation of dichlorprop by Chlorella pyrenoidosa. This work suggests that the enantioselective behaviors of chiral compounds in the environment might be shifted when interactions with other chiral receptors coexist.