Rak-Sakyi transfers to Bramall Lane in pursuit of consistent playing time.
"The plan that the gaffer (Chris Wilder) had for me when we spoke was good to hear, that was why I decided to come," he told the club website.
"It was my first choice, I didn't take in the outside noise, when I spoke to the gaffer I knew it was the right place for me.
"I want to play more football, this is about me getting out and playing as many games as I can."