This paper presents a new design and implementation of a CMOS single-pole-double-throw (SPDT) switch integrated within a transformer balun. The top two metal layers of a CMOS process are used for the primary and secondary inductors of the transformer. The secondary inductor is symmetrically wound, and the CMOS transistors are placed at the center-taps for the switching operation. The transformer is doubly tuned using the transistor parasitic capacitances and an integrated capacitor to overcome the limited coupling factor (k =0.76). The measured insertion loss of the switchable balun is 1.6-1.7 dB at 5-6 GHz for both ports with excellent input and output match, and the input 1-dB power compression point is 12 dBm. The integrated switchable balun occupies 250 × 240 μm2, and can be used for compact low-power T/R applications with a single-ended antenna and differential low-noise and transmit amplifiers. To our knowledge, this is the first implementation of a switchable transformer balun.