The herbarium Trèmols, preserved in the Botanical Institute of Barcelona (IBB), was created during the second half of the 19th century by the Catalan chemist and botanist Frederic Trèmols Borrell (Cadaqués 1831–1900). He was a member of important scientific institutions, including the Real Acadèmia de Ciències i Arts de Barcelona, the Societat Botànica Barcelonesa, the Société Botanique de France, and the Société Helvétique pour l’Échange des Plantes. The value of this herbarium lies in the large volume of specimens that it preserves (12,953) and the high percentage (61.9%) of material of foreign origin that it contains. The Trèmols herbarium was completely digitised in 2019 as part of a wider study that is aimed to classify, digitise, document, review, and, finally, make the IBB historical herbaria available to the scientific community. Herein, we provide a general overview of the almost 13,000 specimens of this collection, which can give valuable insight into the flora that existed more than 100 years ago.