"This endorsement is based on my belief in your leadership and vision and the fact that we need a new message of hope and solidarity of coming together in Washington, D.C.," Napolitano said.
The endorsement is a major gain for Obama in his race against chief foe Clinton. Napolitano, one of several female governors, is the most prominent Democrat in Arizona. Her endorsement could be significant in a state now regarded as winnable by a Democrat after decades as a near-lock for Republicans; the state holds its primary Feb. 5.
Questions from reporters focused on what role Napolitano could conceivably play in an Obama administration.
"I don't want to prejudge or put her on the spot. Let me just say this - I think she is enormously talented ..." Obama said.
Napolitano was elected governor in 2002 and re-elected in 2006. She previously was U.S. attorney for Arizona during most of the Clinton administration and then served a four-year term as the elected state attorney general.
She is regarded as a possible candidate in 2010 for the U.S. Senate seat now held by Republican John McCain, though she has been mentioned as a possible candidate for vice president or for a Cabinet post in a Democratic administration.
She was mentioned in early speculation as a possible running mate for Democratic presidential nominee John Kerry in 2004 and made a prime-time speech at the Democratic National Convention. She is a past chair of the National Governors Association.
Obama also picked up the endorsement of former Sen. Gary Hart, a Colorado Democrat and two-time presidential candidate.
Hart called the Illinois senator "the embodiment of what is best about our nation" and disputed criticism that Obama is inexperienced or lacks national security credentials.
"Senator Obama's personal history uniquely qualifies him to restore America's standing in the world," Hart said in a written release.
Hart lost the 1984 Democratic presidential nomination to former Vice President Walter Mondale. He sought the 1988 nomination before his candidacy was derailed by allegations of an extramarital affair.
