In the protozoa parasite Giardia lamblia, endocytosis and lysosomal protein trafficking are vital parasite-specific processes that involve the action of the adaptor complexes AP-1 and AP-2 and clathrin. In this work, we have identified a single gene in Giardia encoding a protein containing an ENTH domain that defines monomeric adaptor proteins of the epsin family. This domain is present in the epsin or epsin-related (epsinR) adaptor proteins, which are implicated in endocytosis and Golgi-to-endosome protein trafficking, respectively, in other eukaryotic cells. We found that GlENTHp (for G. lamblia ENTH protein) localized in the cytosol, strongly interacted with PI3,4,5P3,was associatedwith the alpha subunit of AP-2, clathrin and ubiquitin andwas involved in receptor-mediated endocytosis. It also bonded PI4P, the gamma subunit of AP-1 and was implicated in ER-to- PV trafficking. Alteration of the GlENTHp function severely affected trophozoite growth showing an unusual accumulation of dense material in the lysosome-like peripheral vacuoles (PVs), indicating that GlENTHp might be implicated in themaintenance of PV homeostasis. In this study,we showed evidence suggesting that GlENTHp might function as a monomeric adaptor protein supporting the findings of other group indicating that GlENTHp might be placed at the beginning of the ENTH family.