Abstract: Orderly molecular self-assembly for tunable micro/nanostructures is aneffective way to prepare novel functional materials with desired properties.Squarylium cyanine (SCy) dyes have received great attention in the fields of laser, imaging, and optoelectronic device. However, the detailed self-assembly behavior of SCy has rarely been investigated. In the present work, two SCy derivatives, D1 and D2 , respectively, bearing four and two carboxylic acid groups at different positions are prepared and used as a model system to investigate the molecular self-assembly, morphology, and optical properties of SCy dyes. The hydrogen-bonding interactions between the carboxylic acid groups in D1 and D2 are determined with X-ray diffraction, 2D nuclear magnetic resonance, and Fourier transformation infrared spectroscopy. The two types of hydrogen bonds in D1 cooperating with inherent π–π stacking interaction result in tunable molecular aggregations, which further leads to the transformation between J-aggregation and H-aggregation of D1 in the solid state in response to ammonia gas. In all, this work provides a feasible and effective way to study the self-assembled aggregates of SCy dyes at both molecular and supramolecular levels, and has developed a reversible sensor for ammonia gas detection.