The subatomic particle with a mass of 1.67 x 10^-27 kg is a proton.

Here's why:

* Protons: Have a mass of approximately 1.6726 x 10^-27 kg.

* Neutrons: Have a mass slightly larger than protons, at about 1.6749 x 10^-27 kg.

* Electrons: Have a much smaller mass, at about 9.1094 x 10^-31 kg.