An overall entomological inventory of the Jean Massart botanical garden was performed in the period 2013-2021. In the frame of this project, supported by "Bruxelles Environnement / Leefmilieu Brussel", the arachnofauna diversity was also studied. This article presents the results of research on the spiders (Araneae), harvestmen (Opiliones), and pseudoscorpions (Pseudoscorpiones) found on this site. A total of 239 species were recorded: 222 spiders, 11 harvestmen and 6 pseudoscorpions. Species lists for all these groups are provided, and some "remarkable" species are briefly discussed. One species of Pseudoscorpiones, Lamprochernes chyzeri (Tömösváry, 1883), is new to the Belgian fauna.