He will join ahead of 2020 and follows Salford team-mate George Griffin whose own move was confirmed yesterday.
Olpherts, 27, has scored eight tries in 20 appearances this term and can also play centre or full-back.
“Derrell has been a standout winger in Super League this season,” said Powell.
“His ability to break tackles and make the game quick is outstanding.
“I am confident he will continue to improve in our system and be a part of a quality back division over the coming seasons.”
The Wakefield-born player started at Dewsbury Rams where he played a handful of matches in 2012 but had a spell out of the game before reappearing at Hemel Stags three years later.
Olpherts then had a prolific two seasons at Newcastle Thunder before getting his chance in Super League at Salford last term at the age of 26.
He becomes Castleford’s third signing for 2020 - Toulouse forward Tyla Hepi is also on his way to Wheldon Road - and hopes to grow his game further.
“I’m a West Yorkshire lad and I grew up watching the Tigers and Wakey so it’ll be good to get back over this side of the Pennines to play,” he said.
“I’m really looking forward to it. I just want to train hard and get a starting position.
“I think that’s what everybody wants to do going into a new club; I want to earn a place in the team week in, week out and help the team achieve their goals.
“Coming up from League One nobody expects you to play Super League but I’ve taken my opportunities when I’ve had them and I’m happy with where I am at the minute.
“I just want to keep getting better and better as the years go on and keep proving people wrong.
“I’ve played centre most of my career and full-back as well, but wing or centre would be my preferred positions and I can play both left and right edge.
“I’ve still got a job to do at Salford first though; we’re in a good position and everybody is fighting for that top-five.”