(S)-(((1-(6-Amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)phosphonic acid, commonly known as (S)-Tenofovir, is a nucleoside phosphonate analog. It is the active enantiomer of the acyclic nucleotide reverse transcriptase inhibitor (NRTI) tenofovir. Its chemical structure features an adenine base linked to a phosphonomethoxypropyl side chain, which mimics the natural substrate dATP but lacks the 3'-hydroxyl group necessary for DNA chain elongation. (S)-Tenofovir is the pharmacologically active component of the prodrugs tenofovir disoproxil fumarate (TDF) and tenofovir alafenamide (TAF). It is a key active pharmaceutical ingredient (API) used in the treatment of chronic hepatitis B virus (HBV) infection and human immunodeficiency virus (HIV) infection. It functions by competitively inhibiting viral reverse transcriptase and, upon incorporation into the growing viral DNA chain, acts as a chain terminator, thereby halting viral replication. As a nucleotide analog, it is also a significant biochemical reagent in virology and antiviral research. It is handled as a potent active substance, requiring appropriate safety precautions in laboratory and manufacturing settings.