The arsenic -specific ACR3 transporter plays pivotal roles in As detoxification in yeast and a group of ancient tracheophytes, the ferns. Despite putative ACR3 genes being present in the genomes of bryophytes, whether they have the same relevance also in this lineage is currently unknown. In this study, we characterized the Mp ACR3 gene from the bryophyte Marchantia polymorpha L. through a multiplicity of functional approaches ranging from phylogenetic reconstruction, expression analysis, loss- and gain -of -function as well as genetic complementation with an Mp ACR3 gene tagged with a fluorescent protein. Genetic complementation demonstrates that Mp ACR3 plays a pivotal role in As tolerance in M. polymorpha , with loss -of -function Mp acr3 mutants being hypersensitive and Mp ACR3 overexpressors more tolerant to As. Additionally, MpACR3 activity regulates intracellular As concentration, affects its speciation and controls the levels of intracellular oxidative stress. The MpACR3::3xCitrine appears to localize at the plasma membrane and possibly in other endomembrane systems. Taken together, these results demonstrate the pivotal function of ACR3 detoxification in both sister lineages of land plants, indicating that it was present in the common ancestor to all embryophytes. We propose that Mp acr3 mutants could be used in developing countries as low-cost and low -technology visual bioindicators to detect As pollution in water.