We demonstrate spin squeezing in a room temperature ensemble of
10¹² Cesium atoms using their internal structure,
where the necessary entanglement is created between nuclear and
electronic spins of each individual atom. This state provides
improvement in measurement sensitivity beyond the standard quantum
limit for quantum memory experiments and applications in quantum
metrology and is thus a complementary alternative to spin
squeezing obtained via inter-atom entanglement. Squeezing of the
collective spin is verified by quantum state tomography.