The 33-year-old, signed by Leeds on a free from Preston North End in 2005 while present Rams boss Billy Davies was in charge at Deepdale, was not expected to feature in today's Premiership game between Derby and Spurs but is likely to be officially unveiled at Pride Park next week.
Lewis is one of the few remaining high earners on the books at Elland Road following a spate of departures this summer, and the lure of the Premiership persuaded him to favour Davies' offer over strong interest from Stoke City.
His appearance at Tranmere Rovers last weekend now looks likely to be his last in a Whites shirt.
The midfielder come left-back made 83 league starts for Leeds, scoring nine goals.
Ben Parker now has an opportunity during the clash with Southend this afternoon, but Leeds are poised to seal a loan move for Wolves' experienced left-back Jamie Clapham.
Wolves boss Mick McCarthy said: "Jamie is going to Leeds, initially on a month's loan."
The Lincoln-born 31-year-old started his career at Tottenham but made just one appearance before moving on to Ipswich Town, Birmingham and then Wolves, where he made 26 appearances last term.