Medicinal plants have been used in traditional medicines all over the world to treat human diseases throughout human history. Many of the medicinal plants have frequently become food and nutrition plants. A more sophisticated investigation resulted in discovering numbers of biologically important secondary metabolites of plants. Pentacyclic triterpenoids represent an important group of the plant secondary metabolites that have emerged as having top biological importance. While the most widespread plant triterpenoids and a majority of their semisynthetic derivatives have been reviewed quite often, other plant pentacyclic triterpenoids and their derivatives have so far been less frequently studied. Therefore, attention has been focused on selected pentacyclic triterpenoids, namely on arjunolic acid, asiatic acid, α- and β-boswellic acids, corosolic acid, maslinic acid, morolic acid, moronic acid, and the friedelane triterpenoids, and on different derivatives of the selected triterpenoids in this review article. A literature search was made in the Web of Science for the given keywords, covering the required area of secondary plant metabolites and their semisynthetic derivatives starting in 2023 and ending in February 2025. The most recently published findings on the biological activity of the selected triterpenoids, and on the structures and the biological activity of their relevant derivatives have been summarized therein. Even if cytotoxicity of the compounds has mainly been reviewed, other biological effects are mentioned if they appeared in the original articles in connection with the selected triterpenoids and their derivatives, listed above. A comparison of the effects of the parent plant products and their derivatives has also been made.