O'Donnell collapsed during the game against Dundee United
The 35-year-old midfielder fell to the ground just as he was about to be replaced with Marc Fitzpatrick near the end of Motherwell's 5-3 victory.
O'Donnell was treated for around five minutes on the field before being taken to a waiting ambulance.
But his death was later confirmed by the Fir Park club.
Motherwell chief executive Ian Stillie said: "The management, directors, players and fans are all stunned and having great difficulty in comprehending what has happened over the last few hours.
"At this stage, we do not have all the facts surrounding the tragic death of Phil O'Donnell.
Phil was a great professional and gave everything when he played the game
Motherwell manager Mark McGhee
"We have been in contact with Phil's wife, Eileen, and his family and they have given us their blessing to speak to the media.
"Phil collapsed during the second-half of the game. He was standing alone at the time.
"He was immediately attended to by Motherwell's club doctor and assisted by Dundee United's club doctor.
"He was transferred almost immediately to an awaiting ambulance and taken to Wishaw General.
"It is believed Phil suffered some form of seizure and was pronounced dead at 1718."
PHIL O'DONNELL'S CAREER 1972 - born in Hamilton 1991 - joins Motherwell 1991 - scores in Scottish Cup final victory over Dundee United 1993 - earns only Scotland cap against Switzerland 1994 - sold to Celtic for a club record £1.75m 1999 - signs for Sheffield Wednesday 2003 - released after injury-hit spell at Wednesday 2004 - returns to Motherwell, becoming club captain
O'Donnell, who earned one cap for Scotland, was lauded as one of the country's brightest talents when he broke into the Motherwell first team as a teenager.
He moved on to Celtic for a club record £1.75m fee, but injury problems were to blight his time there and at his next destination, Sheffield Wednesday.
O'Donnell returned to Motherwell in 2004 and became club captain, playing alongside his nephew, David Clarkson.
Motherwell owner John Boyle said: "Everyone at Motherwell is shocked to the core and we are sure that everyone involved in Scottish football will feel the same.
"Phil was not only an inspirational player for Motherwell and club captain but was an inspirational person."
Motherwell manager Mark McGhee had said immediately after the game that O'Donnell's substitution had been "an absolute coincidence" before his sudden seizure.
The manager had simply been looking to give his captain a rest ahead of Wednesday's game against Hibernian, which has been postponed in light of the player's death.
Clarkson, who had scored two of Motherwell's goals, was substituted moments after O'Donnell's collapse.
MY SPORT: DEBATE Phil, you were a great lad and a great player
TOTT
"We took Clarkson off because he got a fright from the nature of the collapse," said McGhee.
"We are all totally devastated. This is a shocking tragedy."
Motherwell striker Chris Porter added: "His number had come up for him to go off and he literally just passed out.
"He went down and no-one really knew what had happened.
"Doctors and physios ran on, he was unconscious on the floor and it was clear he was in a bad way, but personally I never thought anything would happen.
"Everyone thought he would be okay and he had just fainted, so it was a real shock to find out he had passed away."
Scottish football player Phil O'Donnell has died today after collapsing during a game in which his team Motherwell F.C., of which he was captain, were playing Dundee United in a Scottish Premier League match.

O'Donnell collapsed as his substitution for Marc Fitzpatrick was being arranged. He received treatment for around five minutes before being stretchered off and transferred to a waiting ambulance. David Clarkson, O'Donnell's nephew, was also playing at the time and had scored twice but was taken off due to being visibly distressed by O'Donnel's collapse.

Medics from both teams believed he had suffered a seizure. Attempts were made in the ambulance to revive him, but these failed and he was pronounced dead at 17:18 GMT, having been taken to Wishaw General Hospital.

Born on March 25, 1972 O'Donnell made his debut for Motherwell in 1990, and scored the following year in the final of the Scottish Cup, which Motherwell defeated Dundee United 4-3 in. In 1993 he earned his only international cap for Scotland as a substitute in a World Cup qualification match against Switzerland.

The following year he was bought by Celtic for £1.75 million ($3.5 million). He stayed with them for five years, after which he was transferred to Sheffield Wednesday. A high number of injuries caused Sheffield Wednesday to release him after four years, and the next year, 2004, then Motherwell manager Terry Butcher awarded him an 18-month contract. In 2006 he was appointed captain, and in April the following year, upon the expiration of his original contract, signed a new one as a player-coach.

Motherwell chairman Bill Dickie said "Unfortunately I can confirm very, very sad news that Phil O'Donnell has lost his life. We don't know what it was, but there will be a post-mortem. This is a tragic happening and that's all I can say."

Motherwell owner John Boyle told the press "Everyone at Motherwell is shocked to the core, and we are sure that everyone involved in Scottish football will feel the same. Phil was not only an inspirational player for Motherwell and club captain, but was an inspirational person. All of us at Motherwell are thinking of his wife Eileen and their four children."

Mark McGhee, manager of the team, said "I don't want to say anything more than how devastated everyone at the club is for his wife and his young children. That's what we are all feeling tonight, nothing else matters. Obviously from the club's point of view, we'll gather round to give his family as much support as they need." 

In a press conference given at the hospital, Motherwell chief executive Ian Stillie said "The management, directors, players and fans are all stunned and having great difficulty in comprehending what has happened over the last few hours. At this stage we do not have all the facts surrounding the tragic death of Phil O'Donnell. We have been in contact with Phil's wife Eileen and his family and they have given us their blessing to speak to the media tonight. Phil collapsed during the second half of the game today. He was standing alone at the time. He was immediately attended to by Motherwell's club doctor and assisted by Dundee United's club doctor. He was transferred almost immediately to an awaiting ambulance and taken to Wishaw General. It is believed Phil suffered some form of seizure and was pronounced dead at 1718." 

Gordon Smith, Scottish Football Association chief executive, told reporters "This is absolutely devastating news. Phil was not just a wonderful footballer, he was a great human being. My thoughts are with his family at this tragic time." Meanwhile, Celtic chief executive Peter Lawwell said "When the news came through, everyone at the club was obviously shocked. Our thoughts are with his family to whom we offer extreme condolences for a tragic loss of a young life. Everyone at Celtic Park will mourn him."