Guerreiro began his career in 2015 with the Axel-Merckx-led Axeon Cycling Team – now Hagens Berman Axeon – and spent two seasons there before joining Trek-Segafredo for the 2017 season.
During the first of his two seasons with the American outfit, Guerreiro became the Portuguese road race champion, which remains his last victory.
"Katusha-Alpecin has always been one of my favourite teams," a delighted Guerreiro said in a press release.
"Former Katusha rider Purito Rodríguez [Joaquim Rodríguez, who retired at the end of the 2016 season] has always been one of my idols. There's also a substantial Portuguese connection in the team, which makes it easier for me, of course, although I am used to speaking English after having spent four years in American teams.
"I really look forward to the next step in my career and I feel that Katusha-Alpecin is the right choice."
This season, Guerreiro finished ninth overall and second in the young rider competition at the Tour Down Under, missing out to Team Sky's Egan Bernal, and went on to finish fourth overall at the Herald Sun Tour.
Guerreiro had a strong spring with 14th overall at the Tour of California in May, followed by a top-10 finish at the Tour des Fjords in Norway. More recently, he finished fifth at the Bretagne Classic Ouest-France, where he'd also finished sixth in 2017.
"I've followed Ruben's career for a long time, starting four years ago when he signed with Axel Merckx's team," said Katusha manager Azevedo. "He had some good years with Axel, especially at the Tour of California. I like his style of racing: he's aggressive and always trying to go in the breakaways.
"In the harder moments, he's not someone who tries to hide or to sit in the wheels; he always takes chances, and he's had some good results, like finishing in the top 10 at the Tour Down Under."
Guerreiro is excited to start racing with his new team.
"The Ardennes Classics, one-day races and also week-long stage races are my 'thing'," he said. "Due to some health problems I couldn't always show what I'm capable of, but I will next year. I want to be a good teammate, as well as to win. Joining Katusha-Alpecin offers me that opportunity."