Leah Washington had her leg amputated after two carriages on the Smiler ride collided on 2 June. Three others suffered severe injuries.
Merlin Entertainment said it had contacted those injured and all 16 on board would receive compensation.
Lawyer Paul Paxton, representing three of the families, welcomed the decision.
Ms Washington's boyfriend, 18-year-old Joe Pugh, from Barnsley, is being treated for two broken knees and "extensive" hand injuries at the Royal Stoke University Hospital.
The couple were in the front row of the Smiler ride, alongside Daniel Thorpe and Vicky Balch, when it hit an empty carriage.
Twenty-year-old Ms Balch, from Leyland in Lancashire, has undergone surgery in Stoke and is said to be in a serious, but stable condition.
Mr Thorpe, 27, from Buxton, Derbyshire, is being treated for a collapsed lung and fractured leg at a hospital in Coventry.
Chanda Chauhan, 49, from Wednesbury, who was sitting in the second row of the Smiler, was also admitted to Walsall Manor Hospital with internal injuries.
Merlin Entertainment, which owns Alton Towers, said it had hand-delivered letters to all of those affected.
Last week, chief executive Nick Varney promised there would be no problems with compensation and that those injured would "want for nothing".
Alton Towers reopened to the public on Monday, but the X-Sector area, including the Smiler, remains closed.
The two carriages that collided have been removed by the Health and Safety Executive to be examined in a laboratory and investigations are continuing at the theme park itself.

UK Steel says rising competition and falling prices are damaging firms.
The call for action comes days after Thai steel firm SSI halted production at its Teesside plant due to falling demand, putting 2,000 jobs at risk.
Among other measures, UK Steel wants the government to lower business rates and relax emissions targets.
In a statement it said: "The UK steel sector is vital to the success of manufacturing, employing 30,000 well-paid and highly skilled people, often in areas with higher-than-average unemployment.
"In 2013 it made a Â£9.5bn contribution to the UK economy and had a Â£4.9bn export value."
John Park from the Community trade union told the BBC that it was having trouble getting through to the company's management in Thailand.
"A lot of the things that are happening at SSI are decisions that are being taken thousands of miles away," he said.
"I'm not sure they know themselves exactly what the solution looks like. We would hope that we can find a positive solution to this situation that secures steelmaking on Teesside and supports the community."
During its conference in London on Thursday, UK Steel is expected to outline a number of steps which the government can take in the short-term to "demonstrate its commitment to the UK steel industry".
UK Steel, which is part of the manufacturing trade body EEF, is urging the government to encourage the use of British-produced steel in major infrastructure projects.
It also asked the government to consider relaxing rules on emissions targets for manufacturers - which UK Steel said would cost the steel industry Â£500m by 2019.
A spokesperson for UK Steel told the BBC that the government's business rates are much higher than competitors' in France and Germany - sometimes up to 10 times higher.
UK Steel also said that bills for industries heavily reliant on energy usage were too high and that a review of these rates - announced in the Budget - should be brought forward.
The call comes days after business minister Anna Soubry said that, because of strict EU rules, the government is "limited" in what it can do to help the ailing Teesside steel plant owned by SSI .
About 2,000 jobs are under threat and unions say they have not been told what the current pause in production means.
Contractors who supply workers and equipment to the site have been seen withdrawing their property.
On Wednesday, Stockton South MP James Wharton told the BBC: "At this stage things are still ongoing. Things are very very difficult, I don't want to give false hope about the challenges that are before us for this company. "
Lawyers representing the families of those injured in a rollercoaster crash at Alton Towers have welcomed a promise of compensation.
[NEXT_CONCEPT]
UK steel manufacturers will urge the government to take steps to protect the industry in the face of what they describe as a "perfect storm".