  We find the Yangian symmetry underlying the integrability of type IIB
superstrings on $AdS_3 \times S^3 \times S^3 \times S^1$ with mixed
Ramond-Ramond and Neveu-Schwarz-Neveu-Schwarz flux. The abstract commutation
relations of the Yangian are formulated via RTT realisation, while its matrix
realisation is in an evaluation representation depending on the quantised
coefficient of the Wess-Zumino term. The construction naturally encodes a
secret symmetry of the worldsheet scattering matrix whose generators map
different Yangian levels to each other. We show that in the large effective
string tension limit the Yangian becomes a deformation of a unitary loop
algebra and we derive its universal classical r-matrix.
