The controlled synthesis of high molecular weight branched polyacrylamide (PAM) has been accomplished by using atomic transfer radical polymerization (ATRP) of acrylamide (AM) in water at room temperature. Halogen-functionalized aliphatic polyketones acted as macroinitiators in the polymerization. The obtained branched polymers were used in water solutions to study the effect of the molecular architecture on the rheological properties. For comparison purposes, linear PAM was synthesized by using the same procedure. The intrinsic viscosities and light scattering data suggest that the 13- and 17-arm PAMs are more extended in solution compared to the linear, 4-arm, and 8-arm analogues, at equal total molecular weight. The comparison of linear and 4-, 8-, 12-, 13-, and 17-arm PAM in semidilute solutions demonstrated that the 13- and 17-arm PAM have the highest solution viscosity at equal molecular weight. Depending on the PAM molecular weight and concentration, a significant (as much as 5-fold) increase in solution viscosity (at a shear rate of 10 s(-1)) is observed. The elastic response of aqueous solutions containing the polymers critically depended on the molecular architecture. Both the 4- and 8-arm polymers displayed a larger phase angle value compared to the linear analogue. The 13- and 17-arm PAMs displayed a lower phase angle than the linear one. Ultimately, the rheological properties are dependent on the number of arms present. The combination of a higher hydrodynamic volume and higher entanglement density leads to an improved thickening efficiency (for N >= 13, N being the average number of arms). The improved thickening efficiency of the branched (N >= 13) PAMs makes these polymers highly interesting for application in Enhanced Oil Recovery and drag reduction.