No. First of all there’s already something there called the “Asteroid belt”:
Secondly there’s not enough material there to form a planet.
And lastly if there was something there we’d have discovered it by now. That is after all how we located Neptune.
There’s planets we can see with the naked eye namely Mercury, Venus, Earth as we’re standing on it, Mars, Jupiter, Saturn and Uranus. Uranus is a special case as it’s only visible when at opposition. In other words when we’re at the closest point in our orbit to it and it’s on the same side of the Sun as us. This only occurs once a year. But we “discovered” it in the 18th century and can see it with telescopes.
But as they looked at it they noticed that it wasn’t exactly in the right spot. It was slightly off. We had learned from Newton that gravity affects other objects so that must mean something big was farther away. They did the calculations, worked out it’s location and about 70 years later we discovered Neptune.
Mars and Jupiter are much closer. If there was a planet out there we’d have seen it by now. And if not we’d have detected it through it’s gravity affecting either Mars or some of the asteroids in the belt.
So no. There’s no planet out there between Mars and Jupiter.
