An expansion joint configured to connect first and second vertical mullions of a curtain wall is disclosed. The first and second vertical mullions may each include an elongated body having an inner contour defining a hollow slot, and an inner sleeve may extend through the hollow slot of both of the first and second vertical mullions. The expansion joint may comprise a body, and an inner contour defining a central inner slot configured to receive the inner sleeve therethrough. The expansion joint may further comprise a first raised lip projecting from a lower surface of the body and configured to be inserted inside of the inner contour of the first vertical mullion, and a second raised lip projecting from an upper surface of the body and configured to be inserted inside of the inner contour of the second vertical mullion. The expansion joint may be formed from a polymeric elastomer.