In a series of posts on social media on her 70th birthday, the former Cabinet minister and her relatives vowed to help other people with the disease.
Daughter-in-law Ella, the food blogger known as deliciouslyella, said Baroness Jowell had been diagnosed in May.
In an Instagram post, she wrote: “The last few months have been some of the hardest of our lives, after Matt’s Mum was suddenly diagnosed with brain cancer in May.
“Her bravery, optimism, love and support for others during this process has inspired us both so much, and today, on her 70th birthday, we’re all pledging to try and do everything we can to make people’s lives with cancer better for longer.
“I’m sure so many of you have had to deal with this process too, and Matt and I will work to find new ways of supporting and enhancing people’s lives who have been affected.”
The peer later posted a family picture on Twitter with the message: “Thank you for so much love and support on my birthday. More people living longer better lives with cancer is my birthday pledge.”
Baroness Jowell was one of the Labour party’s best known faces during Tony Blair era and was hailed as the woman who brought the Olympics back to Britain.
The former social worker was elected to Parliament in 1992 and rose swiftly through the Labour ranks, becoming a close ally of former prime minister Tony Blair and one of the leading lights of the new generation of women ministers.
She was appointed public health minister after Labour’s 1997 election triumph - becoming one of the 101 women Labour MPs collectively dubbed “Blair Babes” - and was promoted to employment minister and minister for women in 1999.
She joined the Cabinet as culture secretary in 2001. It was in that role Dame Tessa refused to bow to Civil Service advice that Britain should not bid for the Olympics and also talked round a sceptical Cabinet.
The then-Ms Jowell became Dame Tessa in the 2012 Birthday Honours for political and charitable services and was later made a peer.
Sarah Lindsell, chief executive of The Brain Tumour Charity, said: “Our hearts go out to Tessa Jowell and her family after being diagnosed with a high grade brain tumour.
“And we applaud her pledge on her 70th birthday to help people whose lives are turned upside down by this devastating disease.
“Every year, 11,000 people are diagnosed with a brain tumour - that’s 30 a day - and our five-year research strategy A Cure Can’t Wait aims to double survival within 10 years and halve the harm caused by brain tumours on quality of life.”