Andreea Cristea was rescued from the cold river after the attack, in which British attacker Khalid Masood drove a rented SUV into pedestrians on the bridge and then fatally stabbed an unarmed police officer outside Parliament. Her death brings the death toll from the attack to five, plus Masood, who was shot dead by police.
Cristea's boyfriend Andrei Burnaz suffered a broken foot in the attack.
Her family and Burnaz described Cristea as "our shining ray of light that will forever keep on shining in our hearts," and said they would be donating to charity all the money raised to help her since the March 22 attack.
"After fighting for her life for over two weeks, our beloved and irreplaceable Andreea — wonderful daughter, sister, partner, dedicated friend and the most unique and life-loving person you can imagine — was cruelly and brutally ripped away from our lives in the most heartless and spiritless way," they said in a statement.
They said there are no words to describe the emptiness they feel and thanked medical personnel for their round-the-clock help.
Police said she had been receiving extensive treatment in a London hospital since the attack but that life support was withdrawn Thursday afternoon.
St. Bartholomew's Hospital, where Cristea was treated, said it was "greatly saddened" by her death.
Footage taken at the time showed Cristea falling into the river during the attack. Witnesses saw her lying face-down in the Thames before she was rescued and rushed to an emergency room for emergency surgery after being diagnosed with a blood clot on the brain.
Burnaz needed surgery on his foot but has been discharged from the hospital. He was seen in a wheelchair at a memorial service for victims held last week.
Also killed in the attack were American man Kurt Cochran, 54; Britons Leslie Rhodes, 75, and Aysha Frade, 44; and 48-year-old police officer Keith Palmer.
Dozens of people were wounded.
Police believe 52-year-old Masood, who had convictions for violence, acted alone and was motivated by Islamist extremism.