Erwin, 21, has agreed a three-year-deal at Leeds, with his existing contract at Fir Park due to expire at the end of this month.
Motherwell were keen to keep the young striker and offered him fresh terms, but he has been enticed by the challenge south of the border at Leeds, who yesterday signed young goalkeeper Charlie Horton on a two-year deal from Cardiff City.
The Steelmen are entitled to a development fee for the 6ft 2in Belshill-born forward, whose final game for Motherwell ended in controversy when he got involved in a heated fracas with Rangers rival Bilel Mohsni in a tempestuous SPL play-off game with the Glasgow giants on May 31.
The fee is still to be agreed.
"We are obviously disappointed to lose a player like Lee and I told the lad himself I think he would've benefited from a further year playing week to week in our first-team," said his former boss Ian Baraclough.
"However, with that all being said, I understand how football works and Leeds United is a great club with a great tradition and it's a big opportunity for him."