A search for the lepton-flavor violating decays <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msubsup><a:mi>B</a:mi><a:mi>s</a:mi><a:mn>0</a:mn></a:msubsup><a:mo stretchy="false">→</a:mo><a:mi>ϕ</a:mi><a:msup><a:mi>μ</a:mi><a:mo>±</a:mo></a:msup><a:msup><a:mi>τ</a:mi><a:mo>∓</a:mo></a:msup></a:math> is presented, using a sample of proton-proton collisions at center-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding to a total integrated luminosity of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mn>9</d:mn><d:mtext> </d:mtext><d:mtext> </d:mtext><d:msup><d:mi>fb</d:mi><d:mrow><d:mo>−</d:mo><d:mn>1</d:mn></d:mrow></d:msup></d:math>. The <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mi>τ</f:mi></f:math> leptons are selected using decays with three charged pions. No significant excess is observed, and an upper limit on the branching fraction is determined to be <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mi mathvariant="script">B</h:mi><h:mrow><h:mo stretchy="false">(</h:mo><h:msubsup><h:mi>B</h:mi><h:mi>s</h:mi><h:mn>0</h:mn></h:msubsup><h:mo stretchy="false">→</h:mo><h:mi>ϕ</h:mi><h:msup><h:mi>μ</h:mi><h:mo>±</h:mo></h:msup><h:msup><h:mi>τ</h:mi><h:mo>∓</h:mo></h:msup><h:mo stretchy="false">)</h:mo></h:mrow><h:mo>&lt;</h:mo><h:mn>1.0</h:mn><h:mo>×</h:mo><h:msup><h:mn>10</h:mn><h:mrow><h:mo>−</h:mo><h:mn>5</h:mn></h:mrow></h:msup></h:math> at 90% confidence level. © 2024 CERN, for the LHCb Collaboration 2024 CERN