This paper proposes a flexible, frequency-reconfigurable monopole antenna design with frequency selective surface (FSS) for Internet of Things (IoT) applications. The proposed antenna operates at three of the IoT frequency bands. This antenna is a coplanar waveguide (CPW)-fed monopole with two balanced arms printed on a thin ROGERS 3003 flexible substrate. The length of the right-hand arm of the antenna is used to achieve frequency reconfiguration by using PIN diodes. Three frequency modes of operation have been obtained; the 2.4 GHz frequency band with the right-hand arm is fully truncated, the 3.5 GHz frequency band with the two arms is completely maintained, and the 4 GHz frequency band with the right-hand arm is partially truncated. To improve the gain of the antenna, a simple FSS surface is designed to be placed under the antenna at a distance of 15 mm. The FSS operates efficiently from 2 to 4.5 GHz and has improved the gain of the antenna. A maximum gain of 6.5 dBi, 7.52 dBi, and 7.91 dBi has been achieved at the three frequency bands respectively. The behavior of the flexible antenna has been evaluated in both the flat and bent states, and stable performance has been observed in both cases.