We present the first substellar object in a star-forming region spectroscopically confirmed with Euclid, namely J054158.70-021031.4. This young and faint source is located close to the embedded open cluster NGC 2023. It has a spectral type of L3, a reddening of Av = 3 mag, proper motion consistent with the nearby cluster NGC 2024, and a magnitude of ∼24.5 in the Euclid optical band. At the distance of 403 pc, its inferred mass lies close to the planetary-mass boundary. We discovered the object by searching for ultracool dwarf candidates in the Euclid Early Release Observations. The search was based on photometric criteria, followed by the analysis of slitless 2D spectro-images in order to distinguish features that indicate the nature of these objects. This work demonstrates the potential of Euclid to unveil previously hidden populations of very low-mass objects, located in obscured regions where star formation is ongoing.