It was previously thought there were three states of matter: liquid, solid or gas.
Experts were unsure whether the new state - known as a "chain-melted state" - was its own distinct state of matter or simply atoms transitioning between two stages - whether solid, liquid or gas.
But powerful computer simulations and high-pressure and high-temperature tests carried out on metal potassium have now proven the former to be true.
Several other elements, including sodium, are also capable of existing in this new state, according to experts from the University of Edinburgh.
Dr Andreas Hermann led the study which is published in the journal Proceedings of the National Academy of Sciences, Sky News reported.
He said: "Potassium is one of the simplest metals we know, yet if you squeeze it, it forms very complicated structures. We have shown that this unusual but stable state is part solid and part liquid.
"Recreating this unusual state in other materials could have all kinds of applications."
The work was carried out in collaboration with scientists in China and supported by the European Research Council and the Engineering and Physical Sciences Research Council.