We investigated the adsorption properties of the newly synthesized, hydrophobically functionalized polyelectrolyte (HF-PE), poly(4-styrenesulfonic-co-maleic acid) copolymer (PSS/MA). The hydrophobic alkyl side chains (C12 or C16) were incorporated into the polyelectrolyte backbone via the labile amid linker to obtain the soft HF-PE product with the assumed amount of 15% and 40% degree of grafting for every length of the alkyl chain, i.e., PSS/MA-g-C12NH2 (15% or 40%) as well as PSS/MA-g-C16NH2 (15% or 40%). In the present contribution, we determined both the effect of grafting density and the length of alkyl chain on adsorption at water/air and water/decane interfaces, as well as on top of the polyelectrolyte multilayer (PEM) deposited on a solid surface. The dependence of the interfacial tension on copolymer concentration was investigated by the pendant drop method, while the adsorption at solid surface coated by poly(diallyldimethylammonium chloride)/poly(styrene sulphonate) PEM by the quartz crystal microbalance with dissipation (QCM-D), attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR) and contact angle analysis. We found that surface activity of the hydrophobized copolymer was practically independent of the grafting ratio for C16 side chains, whereas, for C12, the copolymer with a lower grafting ratio seemed to be more surface active. The results of QCM-D and FTIR-ATR experiments confirmed the adsorption of hydrophobized copolymer at PEM along with the modification of water structure at the interface. Finally, it can be concluded that the hydrophobically modified PSS/MA can be successfully applied either as the efficacious emulsifier for the formation of (nano)emulsions for further active substances encapsulation using the sequential adsorption method or as one of the convenient building blocks for the surface modification materials.