We introduce a new family of superalgebras − → B r,s for r, s ≥ 0 such that r + s > 0, which we call the walled Brauer superalgebras, and prove the mixed Scur-Weyl-Sergeev duality for queer Lie superalgebras. More precisely, let q(n) be the queer Lie superalgebra, V = C n|n the natural representation of q(n) and W the dual of V. We prove that, if n ≥ r + s, the superalgebra − → B r,s is isomorphic to the supercentralizer algebra End q(n) (V ⊗r ⊗W ⊗s ) op of the q(n)-action on the mixed tensor space V ⊗r ⊗W ⊗s . As an ingredient for the proof of our main result, we construct a new diagrammatic realization − → D k of the Sergeev superalgebra Ser k . Finally, we give a presentation of − → B r,s in terms of generators and relations.2000 Mathematics Subject Classification. 17B10, 05E10. Key words and phrases. mixed Schur-Weyl-Sergeev duality, queer Lie superalgebras, walled Brauer superalgebras.