The 27 year-old, from Salford, Greater Manchester, smashed the previous record for a wordy phrase by almost 10 seconds.
World's fastest texter Melissa Thompson photo
She wrote ''the razor-toothed piranhas of the genera Serrasalmus and Pygocentrus are the most ferocious freshwater fish in the world. In reality they seldom attack a human'', in a super-fast 25.94 seconds.
The official Guinness World Record is currently held by American Franklin Page, 24, who wrote the same passage in 35.54 seconds in March this year.
The record, which remains subject to Guinness approval, was smashed by Ms Thompson using the Galaxy S's ''SWYPE'' key pad, which enables users to input text without their fingertip leaving the screen.
Ms Thompson, who works for an insurance company, was shopping with her boyfriend, Chris Davies, 23, when they visited a Samsung roadshow and she was invited to have a go at breaking the record.
''I used to send a lot of text messages - 40 or 50-a-day to Chris alone so we both knew I could type fast," she said.
''But since we moved in together and I started my job I haven't been texting as much and, you could say, my fingers were out of shape.
''It's a real shock to find out that I'm the fastest texter in the world.
'But using SWYPE helped. Everyone should give it a go and see how easy it is.''
Ms Thompson used a Samsung's new Galaxy S smartphone when she shaved the 9.6 seconds off Mr Page's record, also achieved on a Samsung device.
The company said it allows people to send text messages at speeds that were never before possible.
The record will now be assessed by the Guiness Book of World Records.