The Index Thomisticus Treebank (IT-TB) is the syntactically annotated portion of the Index Thomisticus corpus (Busa, 1974-1980), which collects the opera omnia of Thomas Aquinas. The IT-TB includes the “analytical” (i.e. surface syntactic) annotation of the entire “Summa contra Gentiles” (4 books), as well as of the concordances of lemma “forma” from “Scriptum super libros sententiarum magistri Petri Lombardi” (entire) and from “Summa Theologiae” (part). In total, the IT-TB consists of more than 450,000 nodes in around 25,000 sentences. The annotation guidelines, inspired by those of the analytical layer of the Prague Dependency Treebank, are available here: http://static.perseus.tufts.edu/docs/guidelines.pdf. The IT-TB features also more than 2.000 sentences from “Summa contra Gentiles” annotated at the tectogrammatical (i.e. underlying syntactic) layer. The annotation guidelines are those of the Prague Dependency Treebank: https://ufal.mff.cuni.cz/pdt2.0/doc/manuals/en/t-layer/pdf/t-man-en.pdf.