✔️ It’s “thank God”.

❌ Thanks God – incorrect

Explanation

“Thanks” is a noun

When we want to thank someone, we often say thanks, meaning “I give you [many] thanks”. Thanks in this case is a plural noun.

“Thank” is a verb

When we say Thank God, or thank goodness, to express relief or gratitude, it literally means “God should be thanked”. Cf. German Gott sei dank. In this case, thank is a verb in the subjunctive. So no “s” at the end please!