A search for quantum black holes in
electron
+
jet
and
muon
+
jet
invariant mass spectra is performed with
140
 
 
fb
−
1
of data collected by the ATLAS detector in proton-proton collisions at
√
s
=
13
 
 
TeV
at the Large Hadron Collider. The observed invariant mass spectrum of
lepton
+
jet
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.