Biomolecular condensates composed of proteins can be formed separately from the liquid cytoplasm by liquid-liquid phase separation (LLPS). The driving forces for LLPS and the physical properties of protein droplets are governed by the amino acid compositions of the constituent proteins. Of the amino acids, the role of arginine vs lysine in LLPS has drawn attention because arginine is prevalent in the phase separation of RNA-binding proteins and is a more potent driver of LLPS compared to other positively charged amino acids, such as lysine. However, what properties of arginine favorably influence LLPS and the physical properties of condensates is still unclear. Here, we demonstrate that arginine can contribute to the hydrophobicity of LLPS through its sp2-hybridized guanidinium group and that hydrophobicity is critical for the physical properties of condensates and temperature-dependent phase behavior. Our findings may provide new insights into the LLPS of stress granule proteins in response to temperature changes in the context of the additional hydrophobic nature of arginine.