Astrophotonics aims to transfer photonic technology to the development of compact astronomical instruments. However, light coupling from a multimode fiber, typically adopted in modern observatories, to a single-mode photonic device still poses a challenge. Though a photonic lantern can enable this transition in a low-loss way, it requires that the number of single-mode fibers (SMFs) at the output is the same as the number of guided modes in the multimode fiber, resulting in a cumbersome fan-out of many single-mode devices to be connected. Herein, we invent an active device in a waveguide form called “the mode detangler” (MD). We show that it can adaptively transform a complex light field from a multimode fiber to a single-mode-like spot. In this way, only one single-mode device is required at the end. The path leading to the idea and the theory behind the mode detangling effect is explained, followed by numerical simulations and experimental demonstrations using a few-mode fiber as proof of concept. We believe this device has the potential to address the multimode-to-single-mode conversion challenge in astrophotonics but also sheds light on (de)multiplexing applications regarding spatial mode technology in optical communications.