Caroline Blower, from Ayr, scooped the top prize when playing the online 75 Ball Bingo Instant Win Game last Friday.
The NHS radiographer said she was stunned when she realised the numbers matched on all four corners of the online Bingo Card.
She said: "I was shocked and screamed. In fact, I phoned the National Lottery almost straight away to make sure I wasn't dreaming and I even didn't tell my family until Monday.
"In four years of playing I've never won anything like this before - it really is a dream come true - a winning try. To be honest I am still in shock, I don't know if it has really sunk in yet."
The 42-year-old, originally from Blackpool, moved to Ayrshire four years ago.
She plans to spend her winnings on her home and watching sport.
She said: "I'd really like to pay off the mortgage and initially buy a new kitchen and bathroom. However, I'm also a big sports fan including of Manchester United and of England's cricket and rugby teams.
"My win will definitely allow me to watch more rugby matches. There's Scotland v England rugby this weekend and one day I may even go and see the Ashes."
The 75 Ball Bingo Instant Win Game is one of the Instant Win Games offered by the National Lottery on its website.
There are 75 balls in the bingo machine, and three ways to play. Players can choose either an instant reveal, have cards marked for them, or mark cards themselves.
The player can win by completing a row, diagonal, column or by matching all four corners, with the top prize of £300,000.