"United is notifying the NRA that we will no longer offer a discounted rate to their annual meeting and we are asking that the NRA remove our information from their website," the airline said on Twitter.
The move came shortly after rival airline Delta announced a similar move, ending the NRA's contract for discounted rates through their group travel programme.
The NRA's website still stated on Saturday that it had contracted special flight discounts with Delta and United, the second- and third-biggest airlines in the world, respectively.
At least seven other major American companies have cut ties with the organisation since the shooting at Marjory Stoneman Douglas High School, where 17 students and staff members were killed by a lone gunman with an assault rifle.
As the largest gun-rights lobbying group in the US, the NRA has become a focal point for those advocating for stricter gun control in the wake of the shooting. Several students survivors have backed calls for a boycott of the organisation, boosting the cause on social media.
"#BoycottTheNRA because our lives shouldn’t be measured in dollars," tweeted student Sarah Chadwick, an outspoken advocate for gun reform.
Classmate David Hogg went farther, saying that politicians who took money from the NRA should be voted out of office.
"If our elected officials are not willing to stand up and say, 'I'm not going to continue to take money from the NRA because children are dying', they shouldn't be in office and they won't be in office because this is a midterm year and this is the change that we need," he told CNN.
Car rental companies like Enterprise and Hertz, and moving companies like Allied Van Lines and North American Van Lines have all ended partnerships with the organisation amid the backlash.
NRA CEO Wayne LaPierre defended the organisation this week at a major conservative political conference, claiming that gun-control advocates wanted to "get rid of the second amendment" and make Americans "less free".
“They don’t care if their laws work or not,” he said in a speech at the Conservative Political Action Conference. "They just want get more laws to get more control over people."
President Donald Trump also defended the organisation on Twitter, saying their employees were "great people and great American patriots" who would "do the right thing".
The president has suggested several gun control measures that put him at odds with the organisation, including raising the minimum age for assault rifle purchases and expanding background checks. He has also suggested arming teachers in order to discourage school shooters.