The 27-year-old moves to Bloomfield Road on a two-year contract, with the club holding the option to extend his stay by a further year.
Letheren will join fellow shot-stopper Colin Doyle in moving to the recently-relegated club when the transfer window opens next month.
The Welshman is thrilled to finally get the deal done. He told the club’s official website: “I’m delighted to get it over the line, it’s been on the back burner for a week now since Neil (McDonald, the Blackpool manager) rang me.
“Since he rang me it’s been the only place that I wanted to come to.
“I’m not just coming here to make up the numbers, I’m coming here to play just like everyone else. I’m sure me and Colin can push each other all the way and that can only be good for Blackpool.”