Is it a sin to get a tattoo or piercing

The one verse that mentions it is Leviticus 19:28: 'Ye shall not... print any marks upon you' - given to Israel amid pagan mourning rituals, in the same chapter that forbids trimming beard corners and wearing mixed-fabric garments (Leviticus 19:19,27).

Christians who eat bacon and wear cotton-polyester have already concluded that Israel's ceremonial boundary markers are not binding on the church (Acts 15; Colossians 2:16-17), so honesty requires treating this verse the same way rather than quoting it selectively. That said, Scripture leaves real questions for the believer to weigh: your body is 'the temple of the Holy Ghost' (1 Corinthians 6:19-20), so what you write on it permanently deserves thought; motive matters (1 Corinthians 10:31 - 'whatsoever ye do, do all to the glory of God'); the content of the tattoo obviously matters; and conscience is not to be steamrolled - 'whatsoever is not of faith is sin' (Romans 14:23), meaning if YOU cannot do it with a clear conscience, for you it would be wrong, while someone else may be free.

Faithful Christians land differently here, and Romans 14 was written for exactly such matters: no despising the inked, no judging the uninked (Romans 14:3-4). It is not the mark on the skin God is reading: 'the LORD looketh on the heart' (1 Samuel 16:7).