The dc ‐impedance of an rf‐biased R‐SQUID is studied analytically for the nonhysteretic regime. A mathematical analysis of the intrinsic behavior of an R‐SQUID, including the mutual interaction between the tank circuit and the SQUID loop, is presented. The results obtained give good agreement with some experimentally observed features. Although the main aim of the present paper was to find an analytic solution for the dc ‐impedance of an R‐SQUID, the results given here can also be used to estimate the R‐SQUID parameters and to optimize the performance of a R‐SQUID noise thermometer. The theory presented here also provides a good basis for modelling the noise properties of a Josephson junction noise thermometer.