There have been several additional planets hypothesized throughout history; however, none of these planets has ever been found.
The most recent of these theorized planets was Planet X, a supposedly giant planet used to explain the deviations from the predicted orbits of Uranus and Neptune. Although Planet X has never been found and scientists are still unable to satisfactorily explain the uranian and neptunian orbital discrepancies, the scientific community has almost unanimously come to the conclusion that Planet X does not exist.
It is highly unlikely that there are any planets beyond the orbit of Pluto.