The paper discusses the problem of rotor flux magnitude estimation for the induction motor (IM) drive and presents a sliding mode (SM) observer that is constructed based on the model of the motor in the synchronous reference frame. To implement the IM control scheme, generally, the magnitude of the rotor flux is needed. If flux regulation is intended, the flux is used in the controller and this generates the d axis reference current. Also, the rotor flux is required if a synchronous current regulation with decoupling compensation is adopted - the flux magnitude is needed to compute the decoupling voltages. Generally, in an IM drive, it is typical to estimate the motor fluxes in the stationary reference frame using an observer and use them for field orientation; then, the flux magnitude can be calculated from the α,β flux components. However, some recently developed observers (that use a speed estimate as an input in the estimation process) do not exactly yield α,β fluxes with accurate magnitude (their phase is accurate though and this allows to obtain the correct field orientation angle). Therefore, in such a case, the flux magnitude needs to be obtained from another source. The paper presents a SM observer for the flux magnitude based on the synchronous reference frame model of the IM. This is developed assuming that the dq axis voltages and currents are available. A speed sensored observer is developed first. Then, this is transformed into a sensorless design by feeding the observer with a speed estimate (which is assumed inaccurate). The paper shows how to design the gains of the observer such that the flux estimation process is insensitive to the speed inaccuracy.