"There's no doubt Semi is one of the best players in the world - a world-class performer," Lam, Bristol's director of rugby, said.
Radradra scored two tries and beat 29 defenders, more than anyone else, in four appearances for Fiji at the World Cup in Japan. Whether playing centre or on the wing, he is considered one of Europe's most lethal attacking threats.
"I'm looking forward to coming to the Premiership and playing under Pat Lam at Bristol," Radradra said. "They're one of the most exciting teams in Europe right now and the way they are doing things is making everybody take notice."
Lam signed a four-year extension with Bristol in May through to 2023 and is steadily building his vision for the Bears, just as he did at Connacht previously.
After four rounds of the Premiership this season Bristol are joint leaders with Chris Boyd's Northampton following three early wins each.
Bristol and Lam, backed by wealthy owner Steve Lansdown, have clearly signalled their intent with Radradra's acquisition.
"Every top club across both codes on the planet wanted him," Lam said. "So for Bristol to be able to bring in a player of his quality underlines the fantastic work that the club is doing.
"Semi has clarity about our vision and knows what the club want to achieve. As a person and a player he's perfect for our culture."