A search for quantum black holes in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>electron</a:mi><a:mo>+</a:mo><a:mtext>jet</a:mtext></a:mrow></a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mi>muon</c:mi><c:mo>+</c:mo><c:mtext>jet</c:mtext></c:mrow></c:math> invariant mass spectra is performed with <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mn>140</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> of data collected by the ATLAS detector in proton-proton collisions at <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msqrt><g:mi>s</g:mi></g:msqrt><g:mo>=</g:mo><g:mn>13</g:mn><g:mtext> </g:mtext><g:mtext> </g:mtext><g:mi>TeV</g:mi></g:math> at the Large Hadron Collider. The observed invariant mass spectrum of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mi>lepton</i:mi><i:mo>+</i:mo><i:mtext>jet</i:mtext></i:mrow></i:math> pairs is consistent with Standard Model expectations. Upper limits are set at 95% confidence level on the production cross section times branching fractions for quantum black holes decaying into a lepton and a quark in a search region with invariant mass above 2.0 TeV. The resulting quantum black hole lower mass threshold limit is 9.2 TeV in the Arkani-Hamed-Dimopoulos-Dvali model, and 6.8 TeV in the Randall-Sundrum model. © 2024 CERN, for the ATLAS Collaboration 2024 CERN