The teenage stand-off became the first signing of the Scottish Qualified programme (SQ), which has broadened its network of scouts across England and abroad to identify players eligible to play for Scotland living outside its borders.
McLelland, 18, turned down a new deal from Castleford to instead switch codes after being spotted by ex-Leeds and Great Britain full-back Alan Tait, who also starred for Scotland and the British Lions.
Tigers’ head coach Daryl Powell said: “Callum had one year left on his contract and was offered an opportunity with the Scottish SQ programme that he felt he couldn’t turn down.
“We made an offer to retain him, but had to accept his wishes on this occasion.
“Having declined an extension from us we felt it was best for both Callum and the club that we allowed him to take up this offer.
“We will use the fee that we received for Callum to retain our best young players as this is a huge focus for Castleford Tigers at the moment.”
McLelland – who qualifies for Scotland through a grandfather born in East Ayrshire – joined Castleford 16 months ago having played union until the age of 13.
He captained England Under-16s and last month scored the match-winning drop goal in England Academy’s 19-18 first Test victory over France.
Scottish Rugby’s technical director Stephen Gemmell said: “We have had a very positive response to the launch of the new SQ Programme and are delighted to be able to attract a young talented rugby league player to Scottish Rugby.
“We have been monitoring Callum through our programme, utilising Alan Tait’s expertise in rugby league, for some time and it’s great he has taken the opportunity to sign for Edinburgh Rugby.”