Nashville police spokesman Don Aaron identified the woman as Sahel Kazemi, whom he called a "friend" of McNair's. She had a single gunshot wound to the head.
Police said the 36-year-old McNair was found on the sofa in the living room, and Kazemi was very close to him on the floor. Aaron said the gun was not "readily apparent" when police first arrived.
Autopsies were planned for today. Aaron said McNair's wife, Mechelle, is "very distraught."
"At this juncture, we do not believe she is involved," he said. "Nothing has been ruled out, but as far as actively looking for a suspect tonight, the answer would be no."
Fred McNair, Steve McNair's oldest brother, said some family members likely will travel to Nashville on Monday to consult with Steve McNair's wife.
"It's still kind of hard to believe," Fred McNair said. "He was the greatest person in the world. He gave back to the community. He loved kids and he wanted to be a role model to kids."
He said he did not know who Kazemi was.
The bodies were discovered Saturday afternoon by McNair's longtime friend Wayne Neeley, who said he rents the condo with McNair.
Aaron said Neeley told authorities he went into the condo, saw McNair on the sofa and Kazemi on the floor but walked first into the kitchen before going back into the living room, where he saw the blood.
Neeley then called a friend, who alerted authorities.
Police said a witness saw McNair arrive at the condo between 1:30 and 2 a.m. Saturday and that Kazemi's vehicle was already there.
Two days ago, Nashville police arrested Kazemi on a DUI charge while driving a 2007 Escalade registered to her and McNair. McNair was in the front seat, but didn't break the law and was allowed to leave by taxi.
McNair was 36 and played 13 seasons before retiring in April 2008. He led Tennessee to the 2000 Super Bowl, where the Titans lost 23-16 to the St. Louis Rams.
In June, McNair opened a restaurant near the Tennessee State University campus.
It was closed Saturday evening, but had become a small memorial, where flowers, candles and notes had been placed outside the door.
On the restaurant's windows were messages: "We will miss you Steve" and "We love you Steve."
A note attached to a small blue teddy bear read, "We will never forget you, Steve. Once a Titan, always a Titan."
McNair was a college football star playing for Alcorn State, a Division I-AA school.
He was a four-time Pro Bowler who played for the Houston Oilers, Tennessee Titans and Baltimore Ravens.
His most noted drive was the last one in the 2000 Super Bowl. He led the Titans 87 yards in the final minute and 48 seconds, only to come up a yard short of the tying touchdown. Kevin Dyson caught his 9-yard pass, but was tackled at the 1-yard line.
"We are saddened and shocked to hear the news of Steve McNair's passing today," Titans owner Bud Adams said in a statement. "He was one of the finest players to play for our organization and one of the most beloved players by our fans. He played with unquestioned heart and leadership and led us to places that we had never reached, including our only Super Bowl."
"If you were going to draw a football player, the physical part, the mental part, everything about being a professional, he is your guy," former Ravens and Titans teammate Samari Rolle said. "I can't even wrap my arms around it. It is a sad, sad day. The world lost a great man today."