Supramolecular polymers form tactoids through liquid–liquid phase separation. This study demonstrates that synthetic supramolecular polymers undergo liquid–liquid phase separation (LLPS) to form highly anisotropic aqueous liquid droplets (tactoids) via an entropy-driven pathway. The process is influenced by the crowding environment, regulated by dextran concentration, which affects the kinetics of supramolecular polymerizations and the properties of LLPS, including phase-separation kinetics, morphology, internal order, fluidity, and mechanical properties of the final tactoids. Substrate–liquid and liquid–liquid interfaces can accelerate LLPS, enabling the formation of a variety of three-dimensional ordered structures, including highly ordered arrays of micrometre-long tactoids at surfaces. The study shows that supramolecular polymerizations can control emerging morphologies, opening up a new field of matter ranging from highly structured aqueous solutions to nanoscopic soft matter. The research highlights the role of LLPS in the formation of membraneless organelles and its significance in biological systems. It also demonstrates that supramolecular polymers, which are non-covalent equivalents of macromolecules, can undergo LLPS, a process previously not observed in synthetic systems. The study uses ureidopyrimidinone glycine (UPy-Gly) and dextran to investigate the LLPS of supramolecular polymers, showing that the process is driven by the entropy of the system and the size and shape of the solutes. The results indicate that the formation of tactoids is influenced by the concentration of dextran, which acts as a macromolecular crowder, and that the phase separation is accelerated by the presence of dextran. The study also explores the effect of substrate–liquid interfaces on LLPS, showing that the process occurs earlier at these interfaces compared to bulk solutions. The formation of highly ordered arrays of vertical tactoids at the interface is attributed to the nucleation of the metastable solution at the interface. The research further demonstrates that the LLPS of supramolecular polymers can be influenced by the presence of other macromolecules, such as alginate, which can promote LLPS by increasing the excluded volume through repulsive interactions. The study provides insights into the dynamics of LLPS, showing that the tactoids can fuse and merge with their neighbors, generating larger structures. The results also indicate that the mechanical properties of the tactoids increase with the concentration of dextran, and that the internal order of the tactoids increases over time. The findings suggest that LLPS of supramolecular polymers can be a general phenomenon for various rod-like supramolecular polymers, and that this process can be controlled by solution conditions, crowding effects, and heterogeneous nucleation at the interface with the substrate. The study highlights the potential of supramolecular polymers in the development of biomaterials for biological, medical, and pharmaceutical applications.Supramolecular polymers form tactoids through liquid–liquid phase separation. This study demonstrates that synthetic supramolecular polymers undergo liquid–liquid phase separation (LLPS) to form highly anisotropic aqueous liquid droplets (tactoids) via an entropy-driven pathway. The process is influenced by the crowding environment, regulated by dextran concentration, which affects the kinetics of supramolecular polymerizations and the properties of LLPS, including phase-separation kinetics, morphology, internal order, fluidity, and mechanical properties of the final tactoids. Substrate–liquid and liquid–liquid interfaces can accelerate LLPS, enabling the formation of a variety of three-dimensional ordered structures, including highly ordered arrays of micrometre-long tactoids at surfaces. The study shows that supramolecular polymerizations can control emerging morphologies, opening up a new field of matter ranging from highly structured aqueous solutions to nanoscopic soft matter. The research highlights the role of LLPS in the formation of membraneless organelles and its significance in biological systems. It also demonstrates that supramolecular polymers, which are non-covalent equivalents of macromolecules, can undergo LLPS, a process previously not observed in synthetic systems. The study uses ureidopyrimidinone glycine (UPy-Gly) and dextran to investigate the LLPS of supramolecular polymers, showing that the process is driven by the entropy of the system and the size and shape of the solutes. The results indicate that the formation of tactoids is influenced by the concentration of dextran, which acts as a macromolecular crowder, and that the phase separation is accelerated by the presence of dextran. The study also explores the effect of substrate–liquid interfaces on LLPS, showing that the process occurs earlier at these interfaces compared to bulk solutions. The formation of highly ordered arrays of vertical tactoids at the interface is attributed to the nucleation of the metastable solution at the interface. The research further demonstrates that the LLPS of supramolecular polymers can be influenced by the presence of other macromolecules, such as alginate, which can promote LLPS by increasing the excluded volume through repulsive interactions. The study provides insights into the dynamics of LLPS, showing that the tactoids can fuse and merge with their neighbors, generating larger structures. The results also indicate that the mechanical properties of the tactoids increase with the concentration of dextran, and that the internal order of the tactoids increases over time. The findings suggest that LLPS of supramolecular polymers can be a general phenomenon for various rod-like supramolecular polymers, and that this process can be controlled by solution conditions, crowding effects, and heterogeneous nucleation at the interface with the substrate. The study highlights the potential of supramolecular polymers in the development of biomaterials for biological, medical, and pharmaceutical applications.