Tempest Domain clerics have the ability Wrath of the Storm (PHB, p. 62):

When a creature within 5 feet of you that you can see hits you with an attack, you can use your reaction to cause the creature to make a Dexterity saving throw. The creature takes 2d8 lightning or thunder damage (your choice) on a failed saving throw, and half as much damage on a successful one.

It doesn't specify what the DC of the saving throw is.

I assume it uses the spell save DC, even though it technically isn't a spell?

Spell save DC = 8 + your proficiency bonus + your Wisdom modifier