The OH Predictive Insights poll found 49 percent support for Biden, who formally entered the race for the White House last month, compared to 44 percent for Trump. Seven percent are undecided.
A similar poll conducted in February showed Biden and Trump tied at 46 points.
Trump, who beat Hillary Clinton in Arizona by 3.5 points in 2016, could be the first Republican to lose the state since Bob Dole lost to Bill Clinton in 1996.
“The Biden bump we have seen in other states carried over to Arizona. Fifteen months before the Arizona primary, Biden is the Democratic candidate best positioned to take on the president,” said Mike Noble, chief of research and managing partner of OH Predictive Insights. “If Democrats put electability above all else, Biden could run away with the nomination and the White House.”