The Northern Irishman fills a hole on the left-wing for the Elland Road outfit.
The Northern Ireland international knows Uwe Rosler from his time at Griffin Park and it was a position Leeds desperately needed to fill.
However, Dallas’ decision to leave Brentford seemed an odd one, considering he played a vital role in taking The Bees to the play-offs last season.
However the club controversially changed managers this season and have already seen Jonathan Douglas leave the club. Moses Odubajo and Andre Gray are also looking likely to leave.
Dallas has given his reasons for departing to getwestlondon, suggesting he wanted guaranteed first-team football and a fresh challenge:
“It was a number of things. You want to play week in week out as much as you can, and I didn't always do that. Plus there were a lot of changes over the summer, with a lot of new faces coming in.
“The way things panned out, the time was right to move on and take on a fresh challenge. Leeds are a big club and it's something I couldn't turn down.”
Leeds were short on wide options last term. Charlie Taylor ended up playing on the left-wing at the end of the season, with fellow full-back Sam Byram on the right.
Byram looks set to remain as a winger for the season, although Taylor was certainly going to be preferred at left-back.
Dallas, who can also operate on the right, is certainly a big signing for the club and Leeds will hope he will improve the side this summer.