Marsters, who played Spike in Buffy the Vampire Slayer, said he was "a big fan" of the Doctor Who spin-off.
And Dale, who used to play Jim Robinson in Neighbours and has recently been in The OC and Ugly Betty in the US, said he was "thrilled" to be in the show.
Filming for the new Torchwood series is currently under way in Cardiff.
The alien-chasing series, starring John Barrowman and Eve Myles, is expected to be shown in 2008 and Marsters is set to guest star in the first episode.
The actor, who starred as a goth vampire in both the cult US series Buffy and its spin-off Angel, said he was "really excited" about his character.
"I can't say too much about him, except he is naughty and a bit of a psychopath," he said.
"I am a huge fan of Doctor Who and Torchwood so it was me who knocked on their door."
And Dale, who has also appeared in Lost and The West Wing, said he was delighted to be back in the UK.
"I am thrilled to be working on Torchwood - I am a huge fan of British television drama."
The cast will also be joined by Freema Agyeman who has been travelling in the tardis as David Tennant's assistant Martha Jones in the last series of Doctor Who.
Agyeman said it was "a huge new challenge" and she could not wait to start filming.
Created by Russell T Davies, the first series of Torchwood introduced a team of investigators based at a secret headquarters in Cardiff Bay and also featured Burn Gorman, Naoko Mori and Gareth David-Lloyd.
