A combination of searches for a new resonance decaying into a Higgs boson pair is presented, using up to 139  fb−1 of 𝑝⁢𝑝 collision data at √𝑠 =13  TeV recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: 𝑏⁢¯𝑏⁢𝑏⁢¯𝑏, 𝑏⁢¯𝑏⁢𝜏+⁢𝜏−, and 𝑏⁢¯𝑏⁢𝛾⁢𝛾. No excess above the expected Standard Model background is observed and upper limits are set at the 95% confidence level on the production cross section of Higgs boson pairs originating from the decay of a narrow scalar resonance with mass in the range 251 GeV–5 TeV. The observed (expected) limits are in the range 0.96–600 fb (1.2–390 fb). The limits are interpreted in the type-I two-Higgs-doublet model and the minimal supersymmetric standard model, and constrain parameter space not previously excluded by other searches.