The M26 is considered a medium tank. However, it is important to keep in mind that classification of tanks is more about which role they are intended to fill than their actual weight. In terms of role the M26 is very much a medium tank, it was an intended replacement for the M4 (another medium tank) and an answer to the german Panther, yet another medium tank.

However, in so far as medium tanks go, the M26 is pretty heavy, especially by allied standards. It is nearly 50% heavier than an M4 and close to twice the weight of a T-34 (which to be fair is ridiculously light for a medium tank). An argument that the US did not need a heavy tank in Europe was made against the development and deployment of the M26 but that did not refer for a heavy tank in terms of role but rather in pure weight. Bridges and roads in Europe were generally not built to handle heavy loads. This did prove to be a problem (at Ludendorf for example) but generally speaking if a Panther could get there, a Pershing could too.