In March-June 2023, we conducted a test-negative study in 1671 children who were hospitalized with acute respiratory illness in Hong Kong. Two hundred and eighty-six children (17.2%) were tested positive for influenza virus including 188 with A(H1N1). We estimated influenza vaccine effectiveness against influenza-associated hospitalization as 69.6% (95% confidence interval: 49.3%, 81.7%).