This paper explores the idealistic elements in modern semiotic studies, with particular recourse to Jakob von Uexküll’s concept of Umwelt. It firstly addresses Uexküll’s affinities with and indebtedness to Kant’s philosophy, pointing out that Uexküll took Kant’s transcendental idealism as a meta-philosophical method to bring forth his own non-mechanistic, constructivist description of an organism’s relationship with its environment and other organisms. Secondly, the paper refines the Uexküllian concept of Umwelt to differentiate the functionally constructed Umwelten of non-human animals from the linguistically and symbolically constructed Umwelten of humans, with the focus on the peculiarity of human language. This is followed by a discussion of the semiotic properties of the human body and its semiotic and symbolic interactions with natural and cultural environments. In the final section, the paper touches on the speculative question of whether an artificial intelligence has an Umwelt. Arguing that the interpreting, meaning-generating capability of a subject plays a significant role in constituting its Umwelt, this paper emphasizes again the idealistic elements that the concept of Umwelt contains within itself. Echoing previous discussions across biosemiotics and anthroposemiotics, the paper aims to contribute to shaping a new understanding of the time-honoured philosophical concept of idealism through the lens of modern semiotic studies.