Lascelles, a centre-back, had been the subject of interest from Queens Park Rangers but has now agreed a long-term move to Newcastle United, with Alan Pardew suggesting he will play a big part in the club's future.
And the Magpies boss offered the same praise to goalkeeper Darlow, telling the club's website: "It is a real coup to have signed two of the finest young talents in English football outside of the Premier League.
"Jamaal and Karl will be a big part of the future of this Club. They have both performed outstandingly in the Championship over the last couple of years and I have no doubt they will be able step up to this level once they return from their loan at Nottingham Forest."
Upon his unveiling, 20-year-old Lascelles said: "As soon as I found out about the move I was over the moon.
"It's a huge step up for me. Newcastle are a club with a massive fanbase, a big history and an unbelievable stadium.
"First, I am going back to Forest for another season and that will be great for my development. Hopefully I can come here next summer an even better player."
Meanwhile, Darlow added: "I'm delighted for it all to be sorted and I can't wait to get started here at Newcastle.
"The loan back is going to be another year but it gives me time to develop my game further before I return. I'm looking forward to the future."