The Brighton striker has become the club's fifth signing of the summer after joining on a season-long loan, with the 20-year-old rebuffing rival Championship interest to head to East Yorkshire.
Longman, who has been at Brighton since the age of 12, impressed on loan at League One outfit AFC Wimbledon last term - scoring nine goals to help the Dons avoid relegation.
Longman's progress has culminated in him penning a new two-year deal at Albion and he has now been allowed out on loan again, with a spell in Championship football viewed as the next barometer in his development.
Nottingham Forest were among a number of rival clubs who were linked with Longman.
On joining Hull, Longman - who can play across the front three - said: "Obviously, there were a few clubs who I have spoken to over the past six weeks. But when I spoke to Grant and I came down and visited the training ground and the stadium, I was really impressed. It is a Premier League standard stadium.
"I think the way Grant presented himself and his philosophies and how he is as a person just drew me in.
"The Wimbledon spell was brilliant. I got everything that I needed to get out of it and got loads of game play and that was the key for me.
"Being young and playing in League One, I wanted to get as many minutes as possible and try and expose myself and push myself to the limits.
"Now that has got me opportunities at clubs like Hull, for instance, to progress my career even further now.
"This is the next step for me and I had long discussions with my agent and Brighton and I think this was the right step. Coming to Hull and meeting everyone and some of the lads, there was a really good atmosphere and I am really looking forward to getting started here."
Longman's arrival follows on from the captures of Nathan Baxter, Randell Williams, George Moncur and Andy Cannon.
McCann added: “We’re absolutely delighted to bring Ryan in. He had a tremendous season last year at AFC Wimbledon and was a top target of ours.
“We wanted to bring in something different to complement our forward players and Ryan will certainly do that.
“He’s got pace, runs in behind and is versatile. We like people who can play in different positions at the top end of the pitch as it gives us better options.
“He comes highly recommended having watched him all season, and the work our data analysts have done shows he will fit in really well with our style of play.
“Knowing how much interest there was, for Ryan to decide to come here is great news for us.”