Annual unpredictable efficacy of vaccines, coupled with emerging drug resistance, underlines the development of new antiviral drugs to treat influenza infections. The N-terminal domain of the PA (PA_N) endonuclease is both highly conserved across influenza strains and serotypes and is indispensable for the viral lifecycle, making it an attractive target for new antiviral therapies. Here, we describe the discovery of a new class of PA_N inhibitors derived from recently identified, highly active hits for PA_N endonuclease inhibition. By use of structure-guided design and systematic SAR exploration, the hits were elaborated through a fragment growing strategy, giving rise to a series of 1, 3-cis-2-substituted-1-(3, 4-dihydroxybenzyl)-6, 7-dihydroxy-1, 2, 3, 4-tetrahydroisoquinoline-3-carboxylic acid derivatives as potent PA_N inhibitors. This approach ultimately resulted in the development of a new lead compound 13e, which exhibited an EC₅₀ value of 4.50 μM against H1N1 influenza virus in MDCK cells.