In this work a macroscopic monofluid theory of liquid helium II, which is based on the extended irreversible thermodynamics, is formulated both in the presence and in the absence of dissipative phenomena. The work is a generalization of previous papers, where the extended thermodynamics of an ideal monoatomic fluid was applied to liquid helium II. It is shown that the behavior of helium II can be described by means of an extended thermodynamic theory where four fields, namely density, temperature, velocity, and heat flux are involved as independent fields. In the presence of dissipative phenomena, constitutive relations for the trace and the deviator of the nonequilibrium stress tensor are determined. In these relations, in addition to the normal viscous terms (which take into account the mechanical dissipation), terms proportional to the gradient of heat flux (which take into account the thermal dissipation) are present. The proposed theory is able to explain the propagation of the two sounds that are typical of helium II, and the attenuation calculated for such sounds is in agreement with the experimental results. Finally, the proposed theory is compared with the two-fluid model making apparent the analogies and the differences.