Cube satellites, aka CubeSats, are a class of tiny satellites that have become popular for space programs. This is because they can be built relatively cheaply using commercial off-the-shelf components. Moreover, CubeSats can communicate with each other, and assemble into swarms to carry out different functions: e.g., wide area measurements and sensing. Swarms of CubeSats also have the effect of increasing the contact period with ground stations allowing for a longer communications window. These capabilities require CubeSats to be equipped with an efficient, high gain, small antenna to facilitate cross-link or inter-satellite communications. Henceforth, this paper presents a high gain coplanar waveguide (CPW)-fed slot antenna for CubeSats. A key feature is the use of a metasurface superstrate structure (MSS) to significantly improve gain and reduce back-lobe emissions. This also has the advantage of minimizing interference to components inside a CubeSat. We have comprehensively evaluated the antenna using the high-frequency simulator structure (HFSS) as well as a carrying out testing on a 3 U (10 x 10 x 30 cm3) CubeSat platform. We have studied the effect of MSS element sets and their position and the effect of a 3-U CubeSat body on the performance of the proposed antenna. The experimental results confirm that our antenna achieves a return loss of 21.5 dB and a fractional impedance bandwidth (BW) of 55.91% with S11 ≤ 10 dB and has a simulated and measured gains of 9.71 and 8.8 dBi respectively at the desired frequency of 2.45 GHz. In contrast, amongst all previous S-band planar antennas that are suitable for CubeSats, the best gain is only 5.96 dB at 2.45 GHz.