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 Sunday.
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."
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.
About 50 people crowded just beyond police tape outside the complex Saturday in the upscale Rutledge Hill neighborhood, some wearing Titans hats. The condominium is located within walking distance of an area filled with restaurants and nightspots, a few blocks from the Cumberland River and within view of the Titans' stadium.
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, a four-time Pro Bowler, led the Titans within a yard of forcing overtime in the 2000 Super Bowl, which they lost 23-16 to the St. Louis Rams. He also played for the Baltimore Ravens before retiring in April 2008.
His most noted drive, the last one in that Super Bowl, came when 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 by the Rams' Mike Jones.
McNair accounted for all of Tennessee's yards in that drive, throwing for 48 yards and rushing for 14. The rest of the yardage came on penalties against the Rams. Before that, he brought the Titans back from a 16-0 deficit to tie the game.
"We don't know the details, but it is a terrible tragedy and our hearts go out to the families involved," NFL commissioner Roger Goodell said in a statement.
"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."
McNair became a nationally known college football star playing for Alcorn State, a Division I-AA school in his home state. His performance in the Southwestern Athletic Conference was so dominant, he became a Heisman Trophy contender and national media flocked to the school in Lorman, Miss., to get look at "Air McNair." He still holds the Division I-AA (now known as Football Championship Subdivision) records for career yards passing (14,496) and total offense (16,823).
McNair began his NFL career in 1995 with the Houston Oilers, who eventually became the Titans, and finished with 31,304 yards passing and 174 touchdowns. McNair played with pain for several years, and the injuries ultimately forced him to retire.
"On the field, there isn't player that was as tough as him, especially at the quarterback position," the Ravens' Derrick Mason said. "What I have seen him play through on the field, and what he dealt with during the week to get ready for a game, I have never known a better teammate."
During a five-game stretch at the end of the 2002 season, McNair was so bruised he couldn't practice. But he started all five games and won them, leading the Titans to an 11-5 finish and a berth in the AFC championship game for the second time in four seasons.
McNair played all 16 games in 2006, his first season in Baltimore, and guided the Ravens to a 13-3 record. But he injured his groin during the season opener last season and never regained the form that put him in those Pro Bowls.
"I am deeply saddened to learn of today's tragic news regarding the death of Steve McNair. He was a player who I admired a great deal," said New England Patriots senior football adviser Floyd Reese, who was GM of the Titans when McNair played for them. "He was a tremendous leader and an absolute warrior. He felt like it was his responsibility to lead by working hard every day, no matter what."
Titans coach Jeff Fisher was out of the country, taking part in the first NFL-USO coaches tour to Iraq.
Ozzie Newsome, Ravens executive vice president and general manager, said he immediately thought of McNair's four sons.
"This is so, so sad. We immediately think of his family, his boys. They are all in our thoughts and prayers," he said "What we admired most about Steve when we played against him was his competitive spirit, and we were lucky enough to have that with us for two years. He is one of the best players in the NFL over the last 20 years."
AP Sports Writer Teresa Walker and Associated Press Writers Randall Paul Dickerson and Lucas L. Johnson II contributed to this report.