Tithonia diversifolia (Asteraceae), is used as traditional medicine in tropical countries for the treatment of various diseases, including malaria. Although numerous studies have assessed the antimalarial properties, nothing is known about the effect of T. diversifolia extracts on trypanosomiasis. In this study extracts of T. diversifolia aerial parts were evaluated for their bioactivity against Trypanosoma brucei. The activity was studied against bloodstream forms of T. brucei (TC221), as well as against mammalian cells (BALB/3T3 mouse fibroblasts), as a counter-screen for toxicity. Both methanolic and aqueous extracts showed significant effects with IC50 values of 1.1 and 2.2 mu g/mL against T. brucei (TC221) and 5.2 and 3.7 mu g/mL against BALB/3T3 cells, respectively. A bioassay-guided fractionation on the methanolic extract yielded in identification of active fractions (F8 and F9) with IC50 values of 0.41 and 0.43 mu g/mL, respectively, against T. brucei (TC221) and 1.4 and 1.5 mu g/mL, respectively, against BALB/3T3 cells,. The phytochemical composition of the extracts and the purified fractions were investigated using HPLC-ESI-MS/MS and 1D and 2D NMR spectra showing the presence of sesquiterpene lactones that in turn were subjected to the isolation procedure. Tagitinin A and C were rather active but the latter presented a very strong inhibition on T. brucei (TC221) with an IC50 value of 0.0042 mu g/mL. This activity was 4.5 times better than that of the reference drug suramin. The results of this study shed light on the antitrypanosomal effects of T. diversifolia extracts and highlighted tagitinin C as one of the possible responsible for this effect. Further structure activity relationships studies on tagitinins are needed to consider this sesquiterpenes as lead compounds for the development of new antitrypanosomal drugs.