Let \(\mathcal {U}(\widehat{\mathfrak {gl}}_{m|n})\) be the universal enveloping superalgebra of \(\widehat{\mathfrak {gl}}_{m|n}\) . There is a natural surjective superalgebra homomorphism \(\xi _r\) from \(\mathcal {U}(\widehat{\mathfrak {gl}}_{m|n})\) to the affine Schur superalgebra \(\widehat{\mathcal {S}}(m|n,r)\) . We give the extra defining relations needed to define the affine Schur superalgebra \(\widehat{\mathcal {S}}(m|n,r)\) as a quotient of \(\mathcal {U}(\widehat{\mathfrak {gl}}_{m|n})\) .