The 19-year-old, known as 'Shabba' by Bluebirds' fans, has joined United on non-contract terms for three months.
He has played three reserve games for the Iron, and has spent the past week training with the first-team.
He arrives from Unibond Premier League side Barrow, where United's FITC officer Lee turnbull is manager, and could be in line for a permanent deal.
Manager Brian Laws said: "We're just going to have a look at him. He needs to work on his fitness, but he's a big strong aggressive lad with plenty of pace.
"We want to get him fit to see what he can really do for the first team"
Rankine played in the 2-1 reserve win over Notts County on Wednesday, and made a good impression on assistant manager Russ Wilcox.
He said: "I thought Rankine gave us that little bit of strength up front which we lacked in the first half. He was the spark that turned the game for us. He's a little bit raw, but he's big, strong and powerful.
"He's quick, and he can get you a goal or two. Ask any centre-half and I don't think they'd want to go up against him."
Barrow manager Lee Turnbull said: "He's signed non-contract terms with Scunthorpe, and he's signed non-contract terms with us.
They weren't prepared to take him on loan, they weren't prepared to pay a transfer fee for him, but they wanted an extended look at him and we've given him the opportunity to do that.
So if he1s not going to be figuring in the Scunthorpe first team squad then he'll play for us."
He scored in Barrow's 1-1 draw with Ashton United last night.
Veteran winger Peter Beagrie has returned to training following a hamstring injury, but is still several weeks away from fitness.