A combination of searches for a new resonance decaying into a Higgs boson pair is presented, using up to <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>139</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:msup><a:mrow><a:mi>fb</a:mi></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>1</a:mn></a:mrow></a:msup></a:mrow></a:math> of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>p</c:mi><c:mi>p</c:mi></c:math> collision data at <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msqrt><e:mi>s</e:mi></e:msqrt><e:mo>=</e:mo><e:mn>13</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>TeV</e:mi></e:math> recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>b</g:mi><g:mover accent="true"><g:mi>b</g:mi><g:mo stretchy="false">¯</g:mo></g:mover><g:mi>b</g:mi><g:mover accent="true"><g:mi>b</g:mi><g:mo stretchy="false">¯</g:mo></g:mover></g:math>, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>b</m:mi><m:mover accent="true"><m:mi>b</m:mi><m:mo stretchy="false">¯</m:mo></m:mover><m:msup><m:mi>τ</m:mi><m:mo>+</m:mo></m:msup><m:msup><m:mi>τ</m:mi><m:mo>−</m:mo></m:msup></m:math>, and <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>b</q:mi><q:mover accent="true"><q:mi>b</q:mi><q:mo stretchy="false">¯</q:mo></q:mover><q:mi>γ</q:mi><q:mi>γ</q:mi></q:math>. 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. © 2024 CERN, for the ATLAS Collaboration 2024 CERN