Fluorescent molecular rotors (FMRs) can act as viscosity sensors in various media including subcellular organelles and microfluidic channels. In FMRs, the rotation of rotators connected to a fluorescent π‐conjugated bridge is suppressed by increasing environmental viscosity, resulting in increasing fluorescence (FL) intensity. In this minireview, we describe recently developed FMRs including push–pull type π‐conjugated chromophores, meso‐phenyl (borondipyrromethene) (BODIPY) derivatives, dioxaborine derivatives, cyanine derivatives, and porphyrin derivatives whose FL mechanism is viscosity‐responsive. In addition, FMR design strategies for addressing various issues (e.g., obtaining high FL contrast, internal FL references, and FL intensity‐contrast trade‐off) and their biological and microfluidic applications are also discussed. Spinning around: This minireview discusses recently developed fluorescent molecular rotors (FMRs), which act as viscosity sensors in various media including subcellular organelles and microfluidic channels. In addition, the mechanism of viscosity‐responsive fluorescence, design strategy of FMRs for addressing remarkable issues, and their various biological and microfluidic applications, are also discussed.