Isothermal titration microcalorimetry, turbidity, and steady-state fluorescence measurements have been used to study interactions of cationic ammonium gemini surfactant (C12C6C12Br2) and single-chain surfactant dodecyltrimethylammonium bromide (DTAB) with anionic polyelectrolytes poly(sodium styrenesulfonates) (NaPSS) and poly(sodium acrylates) (NaPAA) with different molar masses. Without any surfactants, NaPSS with lower molar mass has already self-aggregated into aggregates, whereas NaPAA has no aggregation at any molar mass. All of the polyelectrolytes show a remarkable interaction with the cationic surfactants. Compared with DTAB, C12C6C12Br2 can bind to NaPSS and NaPAA at a very low concentration and has stronger interactions with NaPSS and NaPAA. The flexible NaPAA shows moderately endothermic enthalpies while interacting with the surfactants, but the interaction of the stiff NaPSS with the surfactants exhibits highly exothermic enthalpies. Moreover, the interaction of the stiff NaPSS with the surfactants strongly depends on the polyelectrolyte molar mass, but the polyelectrolyte molar mass almost does not affect the interaction of the flexible NaPAA with the surfactants. Especially, the effect of the polyelectrolyte molar mass becomes more significant when the polyelectrolytes interact with gemini surfactant than with single-chain surfactant. It is revealed that the effects of polyelectrolyte molar mass, chain flexibility, and surfactant architecture on surfactant/polyelectrolyte interactions confine each other.