Tributes from across the political spectrum have poured in, including from former prime ministers Tony Blair and David Cameron.
Baroness Jowell,70, who was diagnosed with a brain tumour in May last year, suffered a haemorrhage on Friday, and had been in a coma until her death on Saturday her family said.
Sharing a photo of her with Baroness Jowell, the Prime Minister tweeted: "The dignity and courage with which Dame Tessa Jowell confronted her illness was humbling and it was inspirational.
"My sympathies to her loving family - Dame Tessa's campaigning on brain cancer research is a lasting tribute to a lifetime of public service."
Former prime minister Tony Blair said: "Tessa had passion, determination and simple human decency in greater measure than any person I have ever known.
"She was the most wise of counsellors, the most loyal and supportive of colleagues, and the best of friends.
"What she achieved was remarkable. She was the first senior politician fully to understand the importance of public health and to shift health policy towards prevention of illness and not only cure.
"She was the instigator of Sure Start and in the process gave hope and opportunity to hundreds of thousands of children.
"She brought the 2012 Olympics and Paralympics to London, and ensured their success."
Labour leader Jeremy Corbyn said he was devastated to hear of Baroness Jowell's death.
In a tweet he wrote: "From Councillor to Cabinet Minister, her achievements were huge, including helping to bring the Olympics to London.
"Her strength in raising awareness of her illness and fighting for better treatment for others inspired us all."
Former prime minister David Cameron tweeted: "Devastated that Tessa Jowell has passed away after her incredibly brave fight. I had the pleasure to work with her on the London Olympics - she was not just a dedicated & passionate campaigner, but a wonderful human being. My heartfelt condolences to her family & friends".
Lord Sebastian Coe, president of the IAAF and former chairman of the London organising committee of the Olympic Games, said: "Tessa was not just a close friend, she was a life enhancer.
"Her contribution to the Olympic and Paralympic Games is easily defined - quite simply, without Tessa there would have been no London 2012, and without Tessa they would not have been the success they were.
"No politician deserves greater credit for the Games. She showed unflinching tenacity in persuading the Prime Minister and the Cabinet that the Government should throw its full weight behind the bid."
Commons Speaker John Bercow said: "Tessa was an indefatigable campaigner who translated care from a word to a deed at every turn.
"Through her focus on Early Years provision, Tessa did more than most to improve lives and promote social justice.
"Passionate, warm and empathetic, she saw the best in everyone and won respect and affection across the political spectrum."
Lord Blunkett said: "Tessa was one of my closest friends for over 40 years.
"Alongside the triumph of helping to win the Olympic Games for London, it will be Tessa as a person who I will remember.
"There when people needed her, both personally and also with her political hat on, and with her bravery over the last year, always thinking of others."
Mr Blair's former Downing Street press secretary Alastair Campbell said on Twitter: "RIP Tessa, wonderful woman, superb politician, the kindest friend."
Sarah Lindsell, chief executive of the Brain Tumour Charity, said: "Tessa Jowell's courage and honesty in speaking about her brain tumour diagnosis, coupled with her fierce determination to improve the lives of others affected by the disease, has already brought hope to an often-forgotten community of patient and families.
"As at so many times in her life, she has been an extraordinary driving force for change. Her passionate support for more flexible clinical trials for brain tumour patients, and for global data-sharing to improve understanding of the disease, will have a real and lasting impact in our quest for a cure."
Jeremy Hunt, the Health Secretary, said he was "devastated" by the loss of Dame Tessa.
He tweeted: "She was quite simply the bravest and the best & brought magic to whatever she touched whether SureStarts, London 2012 or campaigning for those who like her were battling with incurable cancer. RIP."
James Brokenshire, who stepped down as Northern Ireland secretary after being diagnosed with lung cancer, said knowing Baroness Jowell had been a "privilege".
He tweeted: "So very saddened by the news that Tessa Jowell has passed away.
"Tessa's unstinting passion to secure positive change for others even in the face of her own physical adversity is an inspiration to us all."
"A privilege to have known Tessa and I know her message of hope will endure."
Gary Lineker tweeted: "Sorry to hear that dame Tessa Jowell has passed away. A lovely person and admirable politician who played a large part in bringing the Olympic Games to London. RIP."
Former Labour leader Ed Miliband said: "So sad to hear of the death of Tessa Jowell. I will remember her warmth, her passion, her goodness her decency.
"And the extraordinary way she coped with her cancer and tried to use it to help others. The world is a sadder place without Tessa. Deepest condolences to her family."
Food blogger Deliciously Ella, the partner of Baroness Jowell's son Matthew, posted on Instagram: "Matt's extraordinary Mum passed away last night.
"She was lying in Matt, his sister Jess, and their Dad's arms, as they told her that she would live forever in the centre of their souls.
"I've never seen love like I have since becoming part of their family.
"Tessa was the warmest and kindest soul, she welcomed me so generously into their family, and no matter what she has achieved in her life - and she achieved an extraordinary amount, I know her family are the thing that made her most proud.