A combination of searches for the single production of vectorlike top quarks (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>T</a:mi></a:math>) is presented. These analyses are based on proton-proton collisions at <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msqrt><c:mi>s</c:mi></c:msqrt><c:mo>=</c:mo><c:mn>13</c:mn><c:mtext> </c:mtext><c:mtext> </c:mtext><c:mi>TeV</c:mi></c:math> recorded in 2015–2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mn>139</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:msup><e:mi>fb</e:mi><e:mrow><e:mo>−</e:mo><e:mn>1</e:mn></e:mrow></e:msup></e:math>. The <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>T</g:mi></g:math> decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>Z</i:mi></i:math> boson (<k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>T</k:mi><k:mo stretchy="false">→</k:mo><k:mi>H</k:mi><k:mi>t</k:mi></k:math> and <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mrow><n:mi>T</n:mi><n:mo stretchy="false">→</n:mo><n:mi>Z</n:mi><n:mi>t</n:mi></n:mrow></n:math>). The individual searches used in the combination are differentiated by the number of leptons (<q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>e</q:mi></q:math>, <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>μ</s:mi></s:math>) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vectorlike top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vectorlike top quark SU(2) singlet with electroweak coupling, <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi>κ</u:mi></u:math>, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date. © 2025 CERN, for the ATLAS Collaboration 2025 CERN