Dempster-Shafer theory (D-S theory) has been widely used in many fields. Recently, a new entropy called Deng entropy was proposed in D-S theory. As an extension of Shannon entropy, it can deal with uncertainty problems in D-S theory. Entropy originated in physics and was later widely used in many fields. A natural question is what is the form of Deng entropy in physics? In this paper, we proposed the Deng entropy in thermodynamics, and under the conditions of a given system, deduced the Deng entropy in thermodynamics. In addition, we discussed the properties of Deng entropy in thermodynamics. First, the Deng entropy of thermodynamics is an extension of Gibbs entropy, just as Deng entropy is an extension of Shannon’s entropy. Similarly, Deng entropy in thermodynamics is also a measure of uncertainty. Given the state distribution of particles in a system, we can describe the uncertainty of particle states through Deng entropy in thermodynamics. Then, by proof, we find that Deng entropy in thermodynamics does not satisfy additivity. Finally, we also derived the probability distribution corresponding to the system when the Deng entropy in thermodynamics reaches its extreme value.