The 6-foot-8 Smith, who has two years of eligibility left, started on UConn's national championship team as a freshman.
"Roscoe is a versatile player, a terrific defender and rebounder," UNLV coach Dave Rice told the Las Vegas Review Journal. "He's a guy who can score from a lot of different spots on the floor."
Smith is the latest player to leave UConn after the Huskies received a 2013 postseason ban for low Academic Progress Rate scores. Because UConn is ineligible, Smith could play for the Rebels this season, though Rice could not confirm that Friday.
"I do not know the answer to that question," Rice told the Review Journal. "We are going to submit a waiver to the NCAA on his behalf."
Sophomore Jeremy Lamb and freshman center Andre Drummond both announced that they will enter this year's NBA draft, a move both were expected to make even if UConn had been eligible for the tournament.
Earlier this month, UNLV landed ESPN's No. 7 overall senior in power forward Anthony Bennett. UNLV will also add another top-100 recruit, Katin Reinhardt, and Pitt transfer Khem Birch, a former McDonald's All-American.
Smith averaged 4.4 points and 3.4 rebounds in 18.2 minutes a game last season.
Information from The Associated Press was used in this report.