Cops confirmed they were called to a property in Blyth, Northumbria and found Aaron's body.
Armstrong was found dead at his home in the early hours of Tuesday morning.
A spokesman explained, "There is not believed to be any third party involvement and a report will now be prepared for the coroner".
On Monday, he had posted an emotional tribute to his girlfriend on Instagram.
He wrote: "I will never forget that smile I love you so so much baby you're my world forever ever and always".
TV presenter Maya Jama warned about the dangers of online bullying in a heartfelt tribute which said: "Heartbreaking to hear that Sophie Gradon's boyfriend Aaron has taken his life weeks after she passed".
After Sophie's death, believed to be a suicide, Aaron posted messages on his instagram speaking of his grief at her passing.
Our thoughts go out to Aaron's loved ones, and especially the Love Island family.
He posted on Instagram: 'To think I will never see your attractive smile again haunts me you are the girl of my dreams we spoke of having kids and spending the rest of our lives together I'm so devastated that we will never get that chance to grow old together and raise a family.
Friends and family gathered to pay their respects at Sophie's funeral at St George's Church in Jesmond, Newcastle on Thursday 5 July.
He also wrote, "I will see you very soon my angel".
The brunette - who broke Love Island history in 2016 when she became part of the first same-sex couple on the show - went on to describe how despite receiving nice comments, she couldn't help but be drawn to the bad ones.