The subatomic particle found in the nucleus of all isotopes of hydrogen is the proton.

Here's why:

* Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

* Hydrogen has an atomic number of 1, meaning it has one proton in its nucleus.

While different isotopes of hydrogen have varying numbers of neutrons (e.g., deuterium has one neutron, tritium has two neutrons), they all have the same defining characteristic: one proton.