We have synthesized diamondoids using dielectric barrier discharge microplasmas generated inside a microcapillary reactor in supercritical xenon. The plasmas were generated near the critical temperature ( ) and pressure ( ) of xenon in the ranges of and under both batch-type and continuous flow conditions with gas flow rates of 0.01–0.5 mL min−1. Micro-Raman spectra of the synthesized particles showed features characteristic of diamondoids, while gas chromatography–mass spectrometry measurements revealed that diamondoids up to undecamantane were possibly synthesized. Further, the amount of obtained diamantane was greater than those obtained using previously reported diamondoid synthesis processes that involve plasmas in supercritical fluids. This increase is attributed to the higher solubility of the supercritical medium, i.e., xenon, and the higher efficiency of the microreactor. A detailed gas chromatography–mass spectrometry analysis showed that higher diamondoids grow in a stepwise manner via the alternate removal of hydrogen atoms and the addition of methyl groups.