Extensive application of antibiotics leads to their ubiquitous occurrence in coastal aquatic environments. However, it remains largely unknown whether antibiotics can be bioaccumulated and biotransformed in major mariculture organisms such as sea cucumbers and toxicokinetic models for Echinodermata are lacking. In this study, laboratory exposure experiments on juvenile sea cucumber (Apostichopus japonicus) were performed for seven antibiotics (sulfadiazine, sulfamethoxazole, trimethoprim, enrofloxacin, ofloxacin, clarithromycin and azithromycin). Field sea cucumber and surrounding seawater samples were also analyzed using ultra-performance liquid chromatography-tandem mass spectrometry. Results show the sea cucumbers tend to accumulate high concentrations of the antibiotics with kinetic bioconcentration factors (BCFs) up to 1719.7 L·kg-1 for ofloxacin. Significant higher BCFs were observed under lower exposure concentrations. The phenomenon can be explained with a nonlinear adsorption-like mechanism, in which binding sites limit the amount of chemicals adsorbed to the target sites. The BCFs determined in the laboratory agree well with those estimated from the field measurements. Seven biotransformation products (BTPs) of the antibiotics were identified, four of which were not reported previously in aquatic organisms. The BTPs were mainly found in the digestive tract, indicating its high capacity in the biotransformation. A multi-compartmental toxicokinetic model based on principles of passive diffusion was developed, which can successfully predict time-course concentrations of the antibiotics in different compartments of the juvenile sea cucumbers. Sensitivity analysis results showed that the partition coefficients between compartment and the coelomic fluid were the most significant in influencing the predicted internal concentrations, followed by the diffusing capacity coefficients between compartment and seawater. The findings may offer a scientific basis for assessing health risks and guiding healthy mariculture of sea cucumbers.