The Waalian interglacial (German: Waal-Warmzeit or Waal-Interglazial) ^[1] or Waalian Stage was an interglacial period of the Early Pleistocene in north-west Europe. It was preceded by the Eburonian Stage and succeeded by the Menapian Stage. It coincides with part of the much longer Beestonian stage in Britain.^[2] It is variously dated by different authorities. Oxford Reference gives 1.3 to 0.9 million years ago^[3] and the British Geological Survey 1.6 to 1.36 million years ago.^[2] However, the 2020 chart of the international authority on stratigraphic dating, the International Commission on Stratigraphy shows it as c. 1.6 to 1.4 million years ago.^[4] It is distinct from other Pleistocene periods in its complexity, vegetational composition, and vegetational succession.

Its name is derived from a major branch of the Rhine delta, the Waal.

Unlike later interglacial periods, the Waalian Interglacial had at least one period of permafrost conditions, making it a "complex stage".^[5] It has been proposed that the Waalian Interglacial was composed of three phases: a temperate phase, a cool phase, and another temperate phase.^[6][7] The Waalian Interglacial is also distinct from later interglacial periods in that the migration of tree species during this period did not follow a clear pattern of succession (i.e., most of the species that were present at the end of the Waalian were there at the beginning).^[5] In addition, though earlier research indicated that the forest species assemblage of the Waalian period mirrored that of the Tiglian,^[6] more recent research into pollen records show that there was a marked decrease in the number of arboreal taxa from earlier Pleistocene periods.^[8] Common arboreal taxa from the Waalian period included Tsuga, Eucommia, Celtis, and Pterocarya.^[8]