Oganesson, also known as eka-radon or element 118, is the temporary name for the transactinide element having the atomic number 118 and Atomic symbol Og. On the periodic table of the elements, it the last one of the 7th period. Oganesson is currently the only synthetic member of group 18 and has the highest atomic number and highest atomic mass assigned to a discovered element. It has been speculated that ununoctium may be a noble gas, although little is known about it at the moment. It also completes the 7th period.

This element was named after Yuri Oganessian.