From Wikipedia’s post on Uranus:
Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system for the first time in modern history. This was also the first discovery of a planet made using a telescope.
Sir William Herschel was not the first person to observe Uranus – John Flamsteed beat him to it. But Flamsteed did not recognise Uranus for what it was. The credit for the discovery therefore goes to Herschel.