What was the first planet discovered using a telescope?

That would be Uranus, discovered by Sir William Herschel in 1781.

https://www.sciencemuseum.org.uk/objects-and-stories/uranus-first-planet-discovered-telescope

An interesting thing is some of the what are called Galilean moons were discovered using a telescope around 1610, by Galileo - the first celestial bodies found via telescope. But those moons are generally much closer than Uranus is, so despite the rudimentary nature of the telescope they were able to be located.

https://wisconsin.pbslearningmedia.org/resource/ess05.sci.ess.eiu.galileomoon/galileo-discovering-jupiters-moons/
https://science.nasa.gov/solar-system/galileos-observations-of-the-moon-jupiter-venus-and-the-sun/