The given article has attempted to identify the characteristics of the “otherworldly” space as described in Breton lays. The research material consists of two lays by Marie de France, namely, “Guigemar” and “Yonec”, as well as two Middle English lays, “Sir Degaré” and “Sir Orfeo”. All these texts include a motif of a passage into a space that can be described as magical, strange, or even regarded as the underworld. In this paper, this space is defined as “otherworldly”, since this term is broader and more neutral. The article concludes that such space has a number of recurring properties and characteristics, such as unusual desolation, emphasized richness of dwellings, natural markers (forest, river), etc. Some of these properties are also typical of the “otherworldly” spaces described in other medieval genres (such as chivalric romance), while others appear to be specific to lays. The paper also examines the links between the specific features of the “otherworldly” space in Breton lays and Celtic mythology.