A considerable number of reinforced concrete buildings in European countries were built before the 1970s according to rules and principles which are now obsolete, and using reinforcing bars with plain surface that do not guarantee proper bond properties. Most existing studies concerning the characterization of the bond–slip relationship have been carried out on elements built with deformed reinforcing bars. The bond–slip relationship for plain bars is still not well known, and current empirical expressions are found not to take into account all the factors that influence bond–slip performance. This paper presents the results of ten sets of pull-out tests (nine monotonic and one cyclic) performed on specimens built with hot-rolled plain bars. One additional set of specimens built with deformed bars was tested monotonically to highlight the difference, in terms of bond–slip relationship, between plain and deformed rebars. Based on the experimental results, new empirical expressions are proposed for the parameters adopted in one of the more recent bond–slip models for plain bars available in the literature. The new empirical expressions take into account the concrete compressive strength, rebar yield stress, embedded length and bar diameter, and are an improvement on existing expressions. Finally, a new monotonic bond–slip model is proposed, which better represents the bond–slip relationship after the peak strength.