A search for heavy right-handed Majorana neutrinos is performed with the ATLAS detector at the CERN Large Hadron Collider, using the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>140</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mrow><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:mrow></a:math> of proton–proton collision data 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> collected during Run 2. This search targets <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mi>t</e:mi><e:mover accent="true"><e:mrow><e:mi>t</e:mi></e:mrow><e:mrow><e:mo stretchy="false">¯</e:mo></e:mrow></e:mover></e:mrow></e:math> production, in which both top quarks decay into a bottom quark and a <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>W</i:mi></i:math> boson, where one of the <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>W</k:mi></k:math> bosons decays hadronically and the other decays into an electron or muon and a heavy neutral lepton. The heavy neutral lepton is identified through a decay into an electron or muon and another <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>W</m:mi></m:math> boson, resulting in a pair of same-charge same-flavor leptons in the final state. This paper presents the first search for heavy neutral leptons in the mass range of 15–75 GeV using <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>t</o:mi><o:mover accent="true"><o:mi>t</o:mi><o:mo stretchy="false">¯</o:mo></o:mover></o:math> events. No significant excess is observed over the background expectation, and upper limits are placed on the signal cross sections. Assuming a benchmark scenario of the phenomenological type-I seesaw model, these cross section limits are then translated into upper limits on the mixing parameters of the heavy Majorana neutrino with Standard Model neutrinos. © 2024 CERN, for the ATLAS Collaboration 2024 CERN